http://www.gemologyproject.com/wiki/api.php?action=feedcontributions&user=Doos&feedformat=atomThe Gemology Project - User contributions [en]2024-03-28T14:54:06ZUser contributionsMediaWiki 1.28.0http://www.gemologyproject.com/wiki/index.php?title=Amethyst&diff=7717Amethyst2009-03-19T15:01:52Z<p>Doos: /* Polariscope */</p>
<hr />
<div>{{amethyst}}<br />
[[Image:Amethyst.jpg|left|framed|Faceted Amethyst <br /> Photo courtesy of <br />Precision Gems]]<br clear="left" /><br />
{{images}}<br />
<br />
Amethyst is a purple variety of the mineral [[quartz]]. It occurs in all intensities of the color purple from a light pastel to a depth of royal splendor. Until the beginning of the 20th century it was quite rare and costly. When vast deposits were found in Brazil, amethyst became very accessible and affordable. Amethyst has always been linked to the thinking process, ensuring clarity of vision. It inspires creativity, courage and valor.<br />
Amethyst has been successfully synthesized in the lab, so buyers need to be sure their source is qualified to separate natural from lab grown material.<br />
===Magnification===<br />
<br />
Amethyst is a type I stone in the GIA clarity grading system is usually free from eye visible inclusions.<br />
Typical inclusions visible with magnification are:<br />
* liquid feathers "fingerprints" (also known as "tiger stripes" or zebra stripes")<br />
* 2-phase, 3-phase inclusions<br />
* negative crystals<br />
<br />
[[Image:amethyst_fingerprint.jpg|left|thumb|240px|classic "fingerprint" inclusion <br />30X Magnification <br />By Barbra Voltaire]]<br />
<br clear="all" /><br />
<br />
===Polariscope===<br />
<br />
With the use of a [[polariscope]] one can find the typical "bull's eye" for quartz, but that can be seen in both twinned natural stones as in many twinned synthetics.<br />
<br />
To discern the hydrothermal (twinned) synthetics from natural amethyst one must look at the interference figures of the stone without a conoscope. In natural, twinned, amethyst one will see a pattern known as "Brewster fringes". This can be seen when looking down the optic axis of the gem and on lateral rotation this pattern quickly disappears. With the twinned hydrothermal stones these fringes are not seen in the typical triangular form, rather they follow the outline of the stone. On lateral rotation this image will be in view much longer. This method works in most cases, for the remaining 1% to 3% one will need to infrared spectrometry to discern between the two.<br />
<br />
Immersion in a liquid and magnification might aid greatly in spotting the "Brewster fringes".<br />
<br />
==References==<br />
* Notari F., Boillat P.-Y., Grobon C. (2001), Quartz alpha-SiO2: Discrimination des améthystes et des citrines naturelles et synthétiques, Revue de Gemmologie AFG, N° 141/142, pp. 75-80.</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Amethyst&diff=7716Amethyst2009-03-19T14:55:30Z<p>Doos: /* Polariscope */</p>
<hr />
<div>{{amethyst}}<br />
[[Image:Amethyst.jpg|left|framed|Faceted Amethyst <br /> Photo courtesy of <br />Precision Gems]]<br clear="left" /><br />
{{images}}<br />
<br />
Amethyst is a purple variety of the mineral [[quartz]]. It occurs in all intensities of the color purple from a light pastel to a depth of royal splendor. Until the beginning of the 20th century it was quite rare and costly. When vast deposits were found in Brazil, amethyst became very accessible and affordable. Amethyst has always been linked to the thinking process, ensuring clarity of vision. It inspires creativity, courage and valor.<br />
Amethyst has been successfully synthesized in the lab, so buyers need to be sure their source is qualified to separate natural from lab grown material.<br />
===Magnification===<br />
<br />
Amethyst is a type I stone in the GIA clarity grading system is usually free from eye visible inclusions.<br />
Typical inclusions visible with magnification are:<br />
* liquid feathers "fingerprints" (also known as "tiger stripes" or zebra stripes")<br />
* 2-phase, 3-phase inclusions<br />
* negative crystals<br />
<br />
[[Image:amethyst_fingerprint.jpg|left|thumb|240px|classic "fingerprint" inclusion <br />30X Magnification <br />By Barbra Voltaire]]<br />
<br clear="all" /><br />
<br />
===Polariscope===<br />
<br />
With the use of a [[polariscope]] one can find the typical "bull's eye" for quartz, but that can be seen in both twinned natural stones as in many twinned synthetics.<br />
<br />
To discern the hydrothermal (twinned) synthetics from natural amethyst one must look at the interference figures of the stone without a conoscope. In natural, twinned, amethyst one will see a pattern known as "Brewster fringes". This can be seen when looking down the optic axis of the gem and on lateral rotation this pattern quickly disappears. With the twinned hydrothermal stones these fringes are not seen in the typical triangular form, rather they follow the outline of the stone. On lateral rotation this image will be in view much longer. This method works in most cases, for the remaining 1% to 3% one will need to infrared spectrometry to discern between the two.<br />
<br />
Immersion in a liquid and magnification might aid greatly in spotting the "Brewster fringes".</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Amethyst&diff=7715Amethyst2009-03-19T14:54:20Z<p>Doos: /* Polariscope */</p>
<hr />
<div>{{amethyst}}<br />
[[Image:Amethyst.jpg|left|framed|Faceted Amethyst <br /> Photo courtesy of <br />Precision Gems]]<br clear="left" /><br />
{{images}}<br />
<br />
Amethyst is a purple variety of the mineral [[quartz]]. It occurs in all intensities of the color purple from a light pastel to a depth of royal splendor. Until the beginning of the 20th century it was quite rare and costly. When vast deposits were found in Brazil, amethyst became very accessible and affordable. Amethyst has always been linked to the thinking process, ensuring clarity of vision. It inspires creativity, courage and valor.<br />
Amethyst has been successfully synthesized in the lab, so buyers need to be sure their source is qualified to separate natural from lab grown material.<br />
===Magnification===<br />
<br />
Amethyst is a type I stone in the GIA clarity grading system is usually free from eye visible inclusions.<br />
Typical inclusions visible with magnification are:<br />
* liquid feathers "fingerprints" (also known as "tiger stripes" or zebra stripes")<br />
* 2-phase, 3-phase inclusions<br />
* negative crystals<br />
<br />
[[Image:amethyst_fingerprint.jpg|left|thumb|240px|classic "fingerprint" inclusion <br />30X Magnification <br />By Barbra Voltaire]]<br />
<br clear="all" /><br />
<br />
===Polariscope===<br />
<br />
With the use of a [[polariscope]] one can find the typical "bull's eye" for quartz, but that can be seen in both twinned natural stones as in many twinned synthetics.<br />
<br />
To discern the hydrothermal (twinned) synthetics from natural amethyst one must look at the interference figures of the stone without a conoscope. In natural, twinned, amethyst one will see a pattern known as "Brewster fringes". This can be seen when looking down the optic axis of the gem and on lateral rotation this pattern quickly disappears. With the twinned hydrothermal stones these fringes are not seen in the typical triangular form, rather they follow the outline of the stone. On lateral rotation this image will be in view much longer. This method works in most cases, for the remaining 1% to 3% one will need to infrared spectrometry to discern between the two.</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Amethyst&diff=7714Amethyst2009-03-19T14:53:35Z<p>Doos: </p>
<hr />
<div>{{amethyst}}<br />
[[Image:Amethyst.jpg|left|framed|Faceted Amethyst <br /> Photo courtesy of <br />Precision Gems]]<br clear="left" /><br />
{{images}}<br />
<br />
Amethyst is a purple variety of the mineral [[quartz]]. It occurs in all intensities of the color purple from a light pastel to a depth of royal splendor. Until the beginning of the 20th century it was quite rare and costly. When vast deposits were found in Brazil, amethyst became very accessible and affordable. Amethyst has always been linked to the thinking process, ensuring clarity of vision. It inspires creativity, courage and valor.<br />
Amethyst has been successfully synthesized in the lab, so buyers need to be sure their source is qualified to separate natural from lab grown material.<br />
===Magnification===<br />
<br />
Amethyst is a type I stone in the GIA clarity grading system is usually free from eye visible inclusions.<br />
Typical inclusions visible with magnification are:<br />
* liquid feathers "fingerprints" (also known as "tiger stripes" or zebra stripes")<br />
* 2-phase, 3-phase inclusions<br />
* negative crystals<br />
<br />
[[Image:amethyst_fingerprint.jpg|left|thumb|240px|classic "fingerprint" inclusion <br />30X Magnification <br />By Barbra Voltaire]]<br />
<br clear="all" /><br />
<br />
===Polariscope===<br />
<br />
With the use of a [[polariscope]] one can find the typical "bull's eye" for quartz, but that can be seen in both synthetics and many twinned synthetics.<br />
<br />
To discern the hydrothermal (twinned) synthetics from natural amethyst one must look at the interference figures of the stone without a conoscope. In natural, twinned, amethyst one will see a pattern known as "Brewster fringes". This can be seen when looking down the optic axis of the gem and on lateral rotation this pattern quickly disappears. With the twinned hydrothermal stones these fringes are not seen in the typical triangular form, rather they follow the outline of the stone. On lateral rotation this image will be in view much longer. This method works in most cases, for the remaining 1% to 3% one will need to infrared spectrometry to discern between the two.</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Images:Maw-sit-sit&diff=7713Images:Maw-sit-sit2009-03-18T09:47:02Z<p>Doos: </p>
<hr />
<div>[[Image:Rough mawsitsit2.jpeg|thumb|left|240px]]<br />Rough Maw-sit-sit (238 grams)<br />By Scott Davies, american-thai.com<br />
<br clear="all" /><br />
<br />
[[Image:Rough mawsitsit.jpeg|thumb|left|240px]]<br />Rough Maw-sit-sit (649 grams)<br />By Scott Davies, american-thai.com<br />
<br clear="all" /><br />
<br />
[[Image:Mawc2843t1.jpg|thumb|left|240px]]<br />Maw-sit-sit cabochon, 28.43 carats<br />By Scott Davies, american-thai.com<br />
<br clear="all" /><br />
<br />
[[Category:Images|Maw-sit-sit]]</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Images:Alexandrite&diff=7712Images:Alexandrite2009-03-18T09:44:59Z<p>Doos: </p>
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<div>[[Image:Alexc582t1.jpg|thumb|left|400px|Natural Alexandrite Cat's Eyes from South India<br />Photo courtesy of Scott Davies]]</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Alexandrite&diff=7711Alexandrite2009-03-18T09:43:49Z<p>Doos: </p>
<hr />
<div>{{alexandrite}}<br />
<br />
Alexandrite is a phenomenal variety of the mineral [[chrysoberyl]]. Because of the trace amounts of the chromophores vanadium and chromium, alexandrite appears different colors depending on whether it is viewed in natural or incandescent light. In daylight, the stone appears to be green; in artificial light it appears to be raspberry red. It was originally discovered in the Ural Mountains in 1830 on the birthday of Czar Alexander of Russia. Another extraordinary coincidence was that the national colors of Russia were red and green. Today, alexandrite is found in Brazil and to a lesser extent in Africa. Alexandrite has long been associated with great luck and prosperity.<br />
<br />
==Additional Phenomena==<br />
<br />
{{images}}<br />
*Cat's Eye<br />
<br />
[[Image:Alex-blue.gif|left|thumb|250px|Cat's eye Alexandrite under daylight<br />Photo courtesy of The Gem Trader]] <br clear="all" /><br />
[[Image:Alex-purple.gif|left|thumb|250px|Cat's eye Alexandrite under incandescent light<br />Photo courtesy of The Gem Trader]]<br />
<br clear="all" /></div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Feldspar&diff=7710Feldspar2009-03-18T09:36:41Z<p>Doos: /* Plagioclase feldspar */ expanded with info from Scott Davies (id:452)</p>
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<div>{{feldspar}}<br />
<br />
[[Image:Oligoclase feldspar.jpeg|thumb|250px|left|Oligoclase feldspar from Tanzania.<br />Photo courtesy of Scott Davies, americanthai.com]]<br />
<br clear="left" /><br />
<br />
Feldspar is a group of minerals that are very important in rock formation, accounting for over half of Earth's crust! There are a number of varieties that are used in jewelry. The most important are andesine, sunstone, amazonite, moonstone, and labradorite, the latter two known for their phenomenal adularescence and iridescence . <br />
Amazonite was used by the ancient Egyptians for carving images of deities, the stone considered a catalyst between the living and the gods. Moonstone was thought to drive away sleeplessness.<br />
<br />
Feldspars are divided into two types:<br />
* K-feldspars (potassium feldspars)<br />
* Plagioclase feldspars (an isomorphous series between albite and anorthite)<br />
<br />
==K-feldspar==<br />
<br />
K-feldspars grow in monoclinic crystals (except for microcline) and have a chemical composition of KAlSi<sub>3</sub>O<sub>8</sub>.<br /><br />
<br />
===Varieties===<br />
<br />
* Orthoclase<br />
* Orthoclase moonstone<br />
* Microcline ([[amazonite]])<br />
* Sanidine<br />
<br />
==Plagioclase feldspar==<br />
<br />
Plagioclase feldspars grow in triclinic crystals. Its varieties belong to an isomorphous series between albite (NaAlSi<sub>3</sub>O<sub>8</sub>) and anorthite (CaAl<sub>2</sub>Si<sub>2</sub>O<sub>8</sub>).<br />
<br />
===Varieties===<br />
<br />
* Albite (100-90% albite, 0-10% anorthite)<br />
* Oligoclase (90-70% albite, 10-30% anorthite)<br />
* Andesine (70-50% albite, 30-50% anorthite)<br />
* Labradorite (50-30% albite, 50-70% anorthite)<br />
* Bytownite (30-10% albite, 70-90% anorthite)<br />
* Anorthite (10-0% albite, 90-100% anorthite)<br />
<br />
[[image:feldspar.png|framed|left|Timeline image indicating the percentages of the albite-anorthite series]]<br />
<br />
<br clear="all" /><br />
<br />
==="Confetti" sunstone===<br />
"Confetti" sunstone is oligoclase feldspar in composition. The background color can be colorless, yellow, orange, or light green. The colorful flashes are caused by flecks or platelets of hematite. Faceted stones of good size with no fractures and plenty of hematite inclusions are always a treat to see. Tanzania is the only source we know of for this interesting gem.<br />
<br />
<gallery><br />
Image:Suns1537t1.jpg|Confetti sunstone (by Scott Davies)<br />
Image:Suns403t1.jpg|Confetti sunstone (by Scott Davies)<br />
Image:Suns266t1.jpg|Confetti sunstone (by Scott Davies)<br />
</gallery></div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Sea_of_Cortez_pearl&diff=7704Sea of Cortez pearl2009-03-13T09:53:06Z<p>Doos: /* Treatments */</p>
<hr />
<div>{{pearl}}<br />
<br />
[[image:Esfericas3.jpg|thumb|left|250px|Sea of Cortez pearls®<br />Photo Courtesy of Douglas McLaurin Moreno, Perlas del Mar de Cortez]]<br />
<br clear="all" /><br />
<br />
"Perlas del Mar de Cortez" or "Sea of Cortez Pearls" are also known under the commercial name of "Cortez Pearl", a trade-name owned by Columbia Gem House-Tri-Gem Designs. These pearls are produced in the Gulf of California's coastal city of Guaymas, Sonora, Mexico, inside a tranquil Bay known as "Bacochibampo Bay". The pearl farm is owned by "Cultivadores Mexicanos de Perlas S.C.L." since 2004, and was previously owned by the ITESM Education System. It originated as a humble school project (1993) led by four researchers: Enrique Arizmendi Castillo, José Manuel Nava Romo, Douglas McLaurin-Moreno and Sergio Farell Campos. Further funding from ITESM helped to expand this operation into a commercial venture in the year 2000.<br />
<br />
The farm is unique for many reasons, but mainly for being the only pearl farm in the world to utilize a pearl oyster of genus <i>Pteria</i> to produce its cultured pearls. All other marine pearl farms employ <i>Pinctada</i> genus species to grow their pearls.<br />
<br />
==Color==<br />
<br />
The ''pteria sterna'' mollusk is the species used to grow this unusual pearl. The common names for this species are: "Rainbow lipped pearl oyster", "Western Winged Pearl Oyster", "Concha Nácar" and "Callo de árbol". This oyster (Family Pteriidae) is capable of producing pearls in many colors -ranging from white to black, but gray, purple, lavender and blue are more common than white or black- and with varied overtones (such as blue, green, violet, purple and golden).<br />
The pearls are sometimes described as "opalescent" or as "abalone pearls" because of their unusual coloration. This coloration is due to several factors, one being its extremely good nacre coating, and the other one is a series of patterns that form on the surface of a pearl. When seen under a microscope these will resemble a human “finger-print”. Mexican Pearls display two features: an unusually compact structure of its aragonite crystals –seen at 500 nanometers- and the display of “spiraled” sub-patterns, which are unique to these pearls.<br />
<br />
==Shapes==<br />
Round and near-round shapes are exceptionally rare and account for less than 3% of a harvest. Semi-baroque (symmetrical: ovals, drops, buttons) shapes are more common with 25% and baroque (asymmetrical) shapes account for 71% of a harvest.<br />
<br />
==Sizes==<br />
<br />
The average size of the Sea of Cortez pearl is 8.9 mm, ranging in size from 8.3 to 9.8 mm, but they can get as large as 14.3 mm.<br />
Sizes above 10 mm in diameter account for only 5% of a pearl harvest.<br />
<br />
==Nacre Coating==<br />
A minimum of 0.8 mm to a 2.3 mm of nacre coating is expected after the 18-24 month culture period. Pearls with a coating below 0.8 mm are destroyed.<br />
<br />
==Production==<br />
<br />
Both Cultured and Mabe Pearls are produced. Production has been stable at less than 4 kilos per year, due mainly to problems caused by hurricanes but also by choice: High quality production is attained with a controlled harvest. Mabe Pearl production is also limited to some 4-6 thousand per year.<br />
<br />
* 2004: 2.8 kilograms<br />
* 2005: 2.5 kilograms<br />
* 2006: 2.8 kilograms <br />
<br />
With an average weight of 1 gram per pearl, the yearly production is about 2,800 pearls.<br />
<br />
==Treatments==<br />
These pearls are untreated: no polishing, bleaching, irradiation, coating or artificial dyeing is performed on them. After harvest, pearls are soaked in water and then pat dried. The Sea of Cortez Pearl (or “Cortez Pearl”) is the only pearl in the gem industry that completely qualifies under the Fair Trade Gems protocols.<br />
<br />
==Fluorescence==<br />
The shell and pearls of the <i>Pteria sterna</i> oyster display a unique fluorescence under long wave ultraviolet (UV) light, thus this pearl is able of displaying a blood-red to faint-pink glow. All other pearls (regardless of their origin) are inert (do not glow) under this same light.<br />
<br />
==Sources==<br />
<br />
* [http://www.perlas.com.mx/ Sea of Cortez Pearls]</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Sea_of_Cortez_pearl&diff=7703Sea of Cortez pearl2009-03-13T09:51:55Z<p>Doos: updated info from Douglas McLaurin (id: 451)</p>
<hr />
<div>{{pearl}}<br />
<br />
[[image:Esfericas3.jpg|thumb|left|250px|Sea of Cortez pearls®<br />Photo Courtesy of Douglas McLaurin Moreno, Perlas del Mar de Cortez]]<br />
<br clear="all" /><br />
<br />
"Perlas del Mar de Cortez" or "Sea of Cortez Pearls" are also known under the commercial name of "Cortez Pearl", a trade-name owned by Columbia Gem House-Tri-Gem Designs. These pearls are produced in the Gulf of California's coastal city of Guaymas, Sonora, Mexico, inside a tranquil Bay known as "Bacochibampo Bay". The pearl farm is owned by "Cultivadores Mexicanos de Perlas S.C.L." since 2004, and was previously owned by the ITESM Education System. It originated as a humble school project (1993) led by four researchers: Enrique Arizmendi Castillo, José Manuel Nava Romo, Douglas McLaurin-Moreno and Sergio Farell Campos. Further funding from ITESM helped to expand this operation into a commercial venture in the year 2000.<br />
<br />
The farm is unique for many reasons, but mainly for being the only pearl farm in the world to utilize a pearl oyster of genus <i>Pteria</i> to produce its cultured pearls. All other marine pearl farms employ <i>Pinctada</i> genus species to grow their pearls.<br />
<br />
==Color==<br />
<br />
The ''pteria sterna'' mollusk is the species used to grow this unusual pearl. The common names for this species are: "Rainbow lipped pearl oyster", "Western Winged Pearl Oyster", "Concha Nácar" and "Callo de árbol". This oyster (Family Pteriidae) is capable of producing pearls in many colors -ranging from white to black, but gray, purple, lavender and blue are more common than white or black- and with varied overtones (such as blue, green, violet, purple and golden).<br />
The pearls are sometimes described as "opalescent" or as "abalone pearls" because of their unusual coloration. This coloration is due to several factors, one being its extremely good nacre coating, and the other one is a series of patterns that form on the surface of a pearl. When seen under a microscope these will resemble a human “finger-print”. Mexican Pearls display two features: an unusually compact structure of its aragonite crystals –seen at 500 nanometers- and the display of “spiraled” sub-patterns, which are unique to these pearls.<br />
<br />
==Shapes==<br />
Round and near-round shapes are exceptionally rare and account for less than 3% of a harvest. Semi-baroque (symmetrical: ovals, drops, buttons) shapes are more common with 25% and baroque (asymmetrical) shapes account for 71% of a harvest.<br />
<br />
==Sizes==<br />
<br />
The average size of the Sea of Cortez pearl is 8.9 mm, ranging in size from 8.3 to 9.8 mm, but they can get as large as 14.3 mm.<br />
Sizes above 10 mm in diameter account for only 5% of a pearl harvest.<br />
<br />
==Nacre Coating==<br />
A minimum of 0.8 mm to a 2.3 mm of nacre coating is expected after the 18-24 month culture period. Pearls with a coating below 0.8 mm are destroyed.<br />
<br />
==Production==<br />
<br />
Both Cultured and Mabe Pearls are produced. Production has been stable at less than 4 kilos per year, due mainly to problems caused by hurricanes but also by choice: High quality production is attained with a controlled harvest. Mabe Pearl production is also limited to some 4-6 thousand per year.<br />
<br />
* 2004: 2.8 kilograms<br />
* 2005: 2.5 kilograms<br />
* 2006: 2.8 kilograms <br />
<br />
With an average weight of 1 gram per pearl, the yearly production is about 2,800 pearls.<br />
<br />
==Treatments==<br />
These pearls are untreated: no polishing, bleaching, irradiation, coating or artificial dyeing is performed on them. After harvest, pearls are soaked in water and then pat dried. The Sea of Cortez Pearl (or “Cortez Pearl”) is the only pearl in the gem industry that completely qualifies under the Fair Trade Gems protocols.<br />
<br />
Fluorescence<br />
The shell and pearls of the <i>Pteria sterna</i> oyster display a unique fluorescence under long wave ultraviolet (UV) light, thus this pearl is able of displaying a blood-red to faint-pink glow. All other pearls (regardless of their origin) are inert (do not glow) under this same light.<br />
<br />
==Sources==<br />
<br />
* [http://www.perlas.com.mx/ Sea of Cortez Pearls]</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Template:Feldspar&diff=7702Template:Feldspar2009-03-08T16:48:01Z<p>Doos: </p>
<hr />
<div>{| align=right width=250px {{table}}<br />
|-<br />
! colspan=2 | Feldspar<br />
|-<br />
| Chemical composition || Potassium, sodium and calcium-aluminum silicates<br /><br />
I. K-feldspar (KALSi<sub>3</sub>0<sub>8</sub>)<br /><br />
II . Plagioclase feldspar (isomorphous)<br />
|-<br />
| Crystal system || Monoclinic - triclinic<br />
|-<br />
| Habit || Prismatic, often twinned <br />
|-<br />
| Cleavage || Good to perfect<br />
|-<br />
| Fracture || Conchoidal<br />
|-<br />
| Hardness || 6<br />
|-<br />
| Optic nature || Biaxial &plusmn;<br />
|-<br />
| Refractive index || 1.52 - 1.53 (K-feldspar)<br /><br />
1.528 - 1.588 (plagioclase) <br />
|- <br />
| Birefringence || 0.006 - 0.007 (K-feldspar)<br /><br />
0.008 - 0.011 (plagioclase)<br />
|- <br />
| Specific gravity || 2.55 - 2,58 (K-feldspar)<br /><br />
2.60 - 2.80 (plagioclase)<br />
|-<br />
| Lustre || Vitreous<br />
<br />
|}</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Dispersion&diff=7701Dispersion2009-02-28T16:54:32Z<p>Doos: /* Advanced */</p>
<hr />
<div>==Basic==<br />
<br />
[[image:dispersion.png|thumb|300px|right|Dispersion of white light in a prism]]<br />
<br />
Dispersion is the splitting up of white light into its individual wavelengths, what we see as colors. Dispersion occurs with transparent surfaces that are not parallel to each other, such as gemstone facets. Measurement of dispersion is done (in gemology) by calculating the difference of refraction indices for red light waves and violet light waves.<br />
<br />
The source for red light travels at a wavelength of 686.7nm (named the [[Fraunhofer]] B-line) and at 430.8nm for violet light (the Fraunhofer G-line). The interval between red and violet gives the dispersion value of a gemstone.<br />
<br />
All the individual wavelengths have their own refractive index numbers. Red light has a lower refraction index than violet light, thus the violet part of white light will bend more. These values are different for all gemstones, dependent upon the stone's optical density (how fast light can travel inside the gemstone). All transparent gemstones will show dispersion, but the dispersion colors may be masked by the body color of the gemstone. In Diamonds, the color dispersion of white light causes the spectacular "fire" in well-cut brilliant cuts that possess good white color. This "fire" is an interaction between color dispersion and [[brilliance | total internal reflection]].<br />
<br />
[[image:fire.png|thumb|258px|left|"Fire" in Diamond as the result of dispersion and total internal reflection]]<br />
<br />
The refraction index of Diamond (measured with n<sub>D</sub> - or the Fraunhofer D-line) gives a refraction index of 2.417. The value for red light (n<sub>B</sub>) in a Diamond is measured at 2.407 and for violet light (n<sub>G</sub>) it is measured at 2.451. The interval between the B and the G lines is 2.407 - 2.451 = 0.044. Thus, the dispersion value of Diamond is 0.044.<br />
<br />
This example shows that decreasing (shorter) wavelengths have increasing indices of refraction. This is known under the term ''Normal dispersion of the refractive indices''.<br />
<br clear=all><br />
<br />
==Advanced==<br />
<br />
[[image:spectrometerfoto.jpg|thumb|right|300px|Image of a Euromex table spectrometer <br>(Courtesy of Euromex)]]<br />
<br />
Measurement of dispersion is usually carried out using a table spectrometer. Through the [[minimum deviation method]], very accurate refraction indices can be obtained with this apparatus (more accurate than with the refractometer). This type of instrument can be obtained for around USD 1500.00, and takes some skill to operate.<br />
<br />
An easier way to measure dispersion would be to use narrow bandpass interference filters on a refractometer. However, most refractometers are calibrated to take measurments on the sodium D-line, and the B and G lines may be hard to see for most humans.<br />
<br />
Scientists usually measure dispersion between the C and the F lines, giving considerably different values. The values of these lines lay closer to what our eyes can distinguish, so measurements on the C and F lines may be valuable after interpolation to obtain the B and G line values.<br />
<br />
Experimentation with narrow bandpass interference filters with wavelengths of 656nm (n<sub>C</sub>) and 486nm (n<sub>F</sub>) may give good results. One will however need to create a graph with calibration plates for the particular refractometer to correct the errors.<br />
<br />
<br clear=all><br />
<br />
==Related topics==<br />
<br />
* [[Refraction]]<br />
* [[Fraunhofer]]<br />
<br />
==External links==<br />
<br />
* [http://www.australiangemmologist.com.au/gem_dispersion.pdf Practical application for measuring gemstone dispersion on the refractometer]</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Dispersion&diff=7700Dispersion2009-02-28T16:53:52Z<p>Doos: /* Advanced */</p>
<hr />
<div>==Basic==<br />
<br />
[[image:dispersion.png|thumb|300px|right|Dispersion of white light in a prism]]<br />
<br />
Dispersion is the splitting up of white light into its individual wavelengths, what we see as colors. Dispersion occurs with transparent surfaces that are not parallel to each other, such as gemstone facets. Measurement of dispersion is done (in gemology) by calculating the difference of refraction indices for red light waves and violet light waves.<br />
<br />
The source for red light travels at a wavelength of 686.7nm (named the [[Fraunhofer]] B-line) and at 430.8nm for violet light (the Fraunhofer G-line). The interval between red and violet gives the dispersion value of a gemstone.<br />
<br />
All the individual wavelengths have their own refractive index numbers. Red light has a lower refraction index than violet light, thus the violet part of white light will bend more. These values are different for all gemstones, dependent upon the stone's optical density (how fast light can travel inside the gemstone). All transparent gemstones will show dispersion, but the dispersion colors may be masked by the body color of the gemstone. In Diamonds, the color dispersion of white light causes the spectacular "fire" in well-cut brilliant cuts that possess good white color. This "fire" is an interaction between color dispersion and [[brilliance | total internal reflection]].<br />
<br />
[[image:fire.png|thumb|258px|left|"Fire" in Diamond as the result of dispersion and total internal reflection]]<br />
<br />
The refraction index of Diamond (measured with n<sub>D</sub> - or the Fraunhofer D-line) gives a refraction index of 2.417. The value for red light (n<sub>B</sub>) in a Diamond is measured at 2.407 and for violet light (n<sub>G</sub>) it is measured at 2.451. The interval between the B and the G lines is 2.407 - 2.451 = 0.044. Thus, the dispersion value of Diamond is 0.044.<br />
<br />
This example shows that decreasing (shorter) wavelengths have increasing indices of refraction. This is known under the term ''Normal dispersion of the refractive indices''.<br />
<br clear=all><br />
<br />
==Advanced==<br />
<br />
[[image:spectrometerfoto.jpg|thumb|right|300px|Image of a Euromex table spectrometer <br>(Courtesy of Euromex)]]<br />
<br />
Measurement of dispersion is usually carried out using a table spectrometer. Through the [[minimum deviation method]], very accurate refraction indices can be obtained with this apparatus (more accurate than with the refractometer). This type of instrument can be obtained for around USD 1500.00, and takes some skill to operate.<br />
<br />
An easier way to measure dispersion would be to use narrow bandpass interference filters on a refractometer. However, most refractometers are calibrated to take measurments on the sodium D-line, so the B and G lines may be hard to see for most humans.<br />
<br />
Scientists usually measure dispersion between the C and the F lines, giving considerably different values. The values of these lines lay closer to what our eyes can distinguish, so measurements on the C and F lines may be valuable after interpolation to obtain the B and G line values.<br />
<br />
Experimentation with narrow bandpass interference filters with wavelengths of 656nm (n<sub>C</sub>) and 486nm (n<sub>F</sub>) may give good results. One will however need to create a graph with calibration plates for the particular refractometer to correct the errors.<br />
<br />
<br clear=all><br />
<br />
==Related topics==<br />
<br />
* [[Refraction]]<br />
* [[Fraunhofer]]<br />
<br />
==External links==<br />
<br />
* [http://www.australiangemmologist.com.au/gem_dispersion.pdf Practical application for measuring gemstone dispersion on the refractometer]</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Immersion_cell&diff=7699Immersion cell2009-02-26T11:05:37Z<p>Doos: /* Plato method */</p>
<hr />
<div>Immersion cells are use to hold liquids in which a gemstone is placed. The main objectives are to reduce reflections so one can can get a clearer view of the internal features inside the gem, or as a means to estimate the index of refraction of a gemstone through comparising.<br /><br />
Another use is to estimate the index of refraction of an inclusion in a gemstone (the gemstone itself will act as an immersion cell). <br />
__TOC__<br />
<br />
[[Image:Immersion reflection.png|left|250px|thumb|Refraction in a mineral (left) and the lack thereof when immersed in a liquid of equal index of refraction (right).]]<br />
There are circumstances when one can not use the gemological [[refractometer]] but still needs to know the approximate index of [[refraction]] of a gemstone. One method to accomplish this is to compare the gemstone with liquids of known index of refraction.<br /><br />
<br />
When light passes through a mineral the light will be refracted and reflected by the mineral, due to its higher index of refraction compared to air. This makes it possible for us to see the shape of the mineral (color disregarded). If we were to immerse the mineral in a clear liquid that has the same index of refraction as the mineral, the light will not bend inside the mineral, nor will there be any reflection. This causes the mineral to virtually disappear in the liquid.<br />
<br />
The mineral will stand out against its surrounding medium at a certain amount, something we call "relief". The larger the difference in RI between the mineral and its surrounding, the higher its relief (the more clear it will stand out). Not only air or liquids can be used as the surrounding media, a gemstone itself can be that surrounding medium for an inclusion.<br />
<br />
<br clear="all" /><br />
==Refractometry==<br />
<br />
===Anderson method===<br />
<br />
[[Image:Anderson method2.jpg|thumb|300px|right|The Anderson immersion method of comparative refraction.]]<br />
It is no secret that when you insert a drop of water in a glass filled with water, the drop will disappear. Although there are other reasons to why that happens, the focus now will be on the indices of refraction of the drop of water and the water in the glass.<br />
<br />
As the drop has the same index of refraction as the water that is already in the glass, light will travel through it at the same apparent speed. This means that the drop of water will not bend the light more (or less) than the water it is now in (the water filled glass); the drop of water will disappear like magic.<br /><br />
If we were to drop a small glass bead (RI ~ 1.5) in the water (RI ~ 1.3) then there is a difference between the two indices of refraction and you will be able to see the glass bead in the water (although not as clearly as when it was in air (RI ~ 1). The glass bead will refract the light more than the surrounding water and this can be a very useful tool to determine the index of refraction of the glass or any other transparent material like a gemstone.<br />
<br />
It should be clear by now that the closer the indices of refraction between the two media (liquid and gemstone), the more the stone will disappear when it is immersed in the liquid.<br />
<br clear="all" /><br />
<br />
[[Image:Anderson method lens.png|thumb|150px|left|Illustration on the bending of light in a lens.]]<br />
<br />
The famous gemologist B.W. Anderson devised a technique to make this visible with the aid of a series of comparative liquids. This technique is also named the "Anderson immersion method".<br /><br />
When a cut gemstone is placed table down inside a dish of water, the gem will act like a lens if the RI of the gem is larger than its environment (in this case water). Because of the difference in RI between the gemstone and the liquid (not just water) the gemstone will cast an inward shadow on the bottom of the glass. This shadow will be shown as a dark rim around the contour of the gemstone and the wider the rim, the larger the difference in indices of refraction between the stone and the liquid.<br /><br />
By placing a gemstone in a series of liquids with increasing (or decreasing) indices of refraction one can estimate the RI of the stone at hand. '''The finer the rim, the closer the RI of the gemstone matches the RI of the liquid'''.<br />
<br />
When the stone has an index of refraction lower than that of the liquid, the result is reversed and one will see a bright rim around a darker core. <br />
<br />
In order to observe this rim one only needs to insert a small mirror below the glass to redirect the image to the eye. You could of course lift the glass and observe from below, but that would require a ceiling lamp (and in time a physiotherapist).<br /><br />
As the lightsource needs to be above the immersion cell (the glass beaker) it is advisable that the light is diffused before falling on the mirror to reduce reflections from the light source. For this purpose one can use an immersion cell with a frosted bottom or create the diffusion by a thin sheet of white toilet paper (peeled).<br />
<br clear="all" /><br />
<br />
{| style="border: solid black 0px;" align="center" width="420" <br />
|-<br />
|align="center"|<br />
[[Image:Anderson rims.png|400px|thumb|From left to right: casted rims (shadows) by a brilliant cut gemstone for decreasing RI difference between liquid and gemstone. From thick to fine.]]<br />
|}<br />
<br />
The thickness of the rim should always be judged in relation to the size of the gemstone. In other words the shadow is a percentage of the diameter of the gemstone.<br /><br />
When the immersion liquid is water it is obvious that a diamond will show a relative thick rim, while a sapphire will cast a lesser thick one. If you put two of these stones next to eachother in an immersion cell (a small diamond and a large sapphire), odds are that the absolute thickness of the casted shadows are the same or similar. For that reason one needs to estimate the rims in relation to the diameter of the stones.<br />
<br />
There are many liquids one can use and in cases that this technique needs to be applied to estimate the refractive index of a gemstone, it is best to immerse the stone in several liquids with progressive RI values. The RI of the stone will lie between the RI values of the liquids in which the stone casts the smallest shadows. In cases where there is no casted shadow, the RI of the liquid is equal to the RI of the gem (like the drop of water in a glass of water).<br />
<br />
===Plato method===<br />
<br />
[[Image:Plato immersion.png|left|frame|Idealized illustration of the "Becke lines" in the Plato immersion technique.]]<br />
<br />
Like the Anderson immersion method, the Plato method (named after W. Plato) is an immersion technique to compare a gemstone with a liquid of known index of refraction. Both methods are derivations of the Becke line method that is most often used by mineralogists.<br /><br />
As with the Becke line technique, the Plato method requires magnification with a microscope.<br />
<br />
A stone is immersed in a liquid of know index of refraction and the immersion dish is placed on the microscope stage with transmitted light. The diaphragm is closed to approximate the diameter of the stone and the focus of the lenses is just above the stone. When the facets edges of the stone appear dark, the index of refraction of the liquid is higher than that of the stone. On focusing inside the stone, the facet edges start to appear bright.<br />
<br />
When the reverse happens (bright on focus above the stone and dark when the focus is lowered into the stone), the stone has a lower index of refraction compared to the liquid. When the RI of the liquid is equal (or almost equal) to the RI of the stone, the edges will remain bright.<br />
<br />
Although this technique is fairly straightforward, it does require some practice with stones and liquids of known indices of refraction. Especially trying to focus just above the stone can be cumbersome as the image of the stone is very blurry at that point. On lowering the focus inside the stone more facet edges will start to appear that can provide the information one is searching for.<br />
<br />
This technique should not be confused with the color concentration on facet edges in some diffused stones as sapphire.<br />
<br clear="all" /><br />
<br />
==References==<br />
<br />
* ''Guide to Affordable Gemology'' (2001) - Dr. W. Wm. Hanneman<br />
* ''Gemmology'' 3rd edition (2005) - Peter G. Read ISBN 0750664495<br />
* ''Gems Their Sources, Descriptions and Identification'' 4th Edition (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''Introduction to Optical mineralogy'' 2004 - William D. Nesse ISBN 0195149106</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Immersion_cell&diff=7698Immersion cell2009-02-26T11:02:37Z<p>Doos: /* References */</p>
<hr />
<div>Immersion cells are use to hold liquids in which a gemstone is placed. The main objectives are to reduce reflections so one can can get a clearer view of the internal features inside the gem, or as a means to estimate the index of refraction of a gemstone through comparising.<br /><br />
Another use is to estimate the index of refraction of an inclusion in a gemstone (the gemstone itself will act as an immersion cell). <br />
__TOC__<br />
<br />
[[Image:Immersion reflection.png|left|250px|thumb|Refraction in a mineral (left) and the lack thereof when immersed in a liquid of equal index of refraction (right).]]<br />
There are circumstances when one can not use the gemological [[refractometer]] but still needs to know the approximate index of [[refraction]] of a gemstone. One method to accomplish this is to compare the gemstone with liquids of known index of refraction.<br /><br />
<br />
When light passes through a mineral the light will be refracted and reflected by the mineral, due to its higher index of refraction compared to air. This makes it possible for us to see the shape of the mineral (color disregarded). If we were to immerse the mineral in a clear liquid that has the same index of refraction as the mineral, the light will not bend inside the mineral, nor will there be any reflection. This causes the mineral to virtually disappear in the liquid.<br />
<br />
The mineral will stand out against its surrounding medium at a certain amount, something we call "relief". The larger the difference in RI between the mineral and its surrounding, the higher its relief (the more clear it will stand out). Not only air or liquids can be used as the surrounding media, a gemstone itself can be that surrounding medium for an inclusion.<br />
<br />
<br clear="all" /><br />
==Refractometry==<br />
<br />
===Anderson method===<br />
<br />
[[Image:Anderson method2.jpg|thumb|300px|right|The Anderson immersion method of comparative refraction.]]<br />
It is no secret that when you insert a drop of water in a glass filled with water, the drop will disappear. Although there are other reasons to why that happens, the focus now will be on the indices of refraction of the drop of water and the water in the glass.<br />
<br />
As the drop has the same index of refraction as the water that is already in the glass, light will travel through it at the same apparent speed. This means that the drop of water will not bend the light more (or less) than the water it is now in (the water filled glass); the drop of water will disappear like magic.<br /><br />
If we were to drop a small glass bead (RI ~ 1.5) in the water (RI ~ 1.3) then there is a difference between the two indices of refraction and you will be able to see the glass bead in the water (although not as clearly as when it was in air (RI ~ 1). The glass bead will refract the light more than the surrounding water and this can be a very useful tool to determine the index of refraction of the glass or any other transparent material like a gemstone.<br />
<br />
It should be clear by now that the closer the indices of refraction between the two media (liquid and gemstone), the more the stone will disappear when it is immersed in the liquid.<br />
<br clear="all" /><br />
<br />
[[Image:Anderson method lens.png|thumb|150px|left|Illustration on the bending of light in a lens.]]<br />
<br />
The famous gemologist B.W. Anderson devised a technique to make this visible with the aid of a series of comparative liquids. This technique is also named the "Anderson immersion method".<br /><br />
When a cut gemstone is placed table down inside a dish of water, the gem will act like a lens if the RI of the gem is larger than its environment (in this case water). Because of the difference in RI between the gemstone and the liquid (not just water) the gemstone will cast an inward shadow on the bottom of the glass. This shadow will be shown as a dark rim around the contour of the gemstone and the wider the rim, the larger the difference in indices of refraction between the stone and the liquid.<br /><br />
By placing a gemstone in a series of liquids with increasing (or decreasing) indices of refraction one can estimate the RI of the stone at hand. '''The finer the rim, the closer the RI of the gemstone matches the RI of the liquid'''.<br />
<br />
When the stone has an index of refraction lower than that of the liquid, the result is reversed and one will see a bright rim around a darker core. <br />
<br />
In order to observe this rim one only needs to insert a small mirror below the glass to redirect the image to the eye. You could of course lift the glass and observe from below, but that would require a ceiling lamp (and in time a physiotherapist).<br /><br />
As the lightsource needs to be above the immersion cell (the glass beaker) it is advisable that the light is diffused before falling on the mirror to reduce reflections from the light source. For this purpose one can use an immersion cell with a frosted bottom or create the diffusion by a thin sheet of white toilet paper (peeled).<br />
<br clear="all" /><br />
<br />
{| style="border: solid black 0px;" align="center" width="420" <br />
|-<br />
|align="center"|<br />
[[Image:Anderson rims.png|400px|thumb|From left to right: casted rims (shadows) by a brilliant cut gemstone for decreasing RI difference between liquid and gemstone. From thick to fine.]]<br />
|}<br />
<br />
The thickness of the rim should always be judged in relation to the size of the gemstone. In other words the shadow is a percentage of the diameter of the gemstone.<br /><br />
When the immersion liquid is water it is obvious that a diamond will show a relative thick rim, while a sapphire will cast a lesser thick one. If you put two of these stones next to eachother in an immersion cell (a small diamond and a large sapphire), odds are that the absolute thickness of the casted shadows are the same or similar. For that reason one needs to estimate the rims in relation to the diameter of the stones.<br />
<br />
There are many liquids one can use and in cases that this technique needs to be applied to estimate the refractive index of a gemstone, it is best to immerse the stone in several liquids with progressive RI values. The RI of the stone will lie between the RI values of the liquids in which the stone casts the smallest shadows. In cases where there is no casted shadow, the RI of the liquid is equal to the RI of the gem (like the drop of water in a glass of water).<br />
<br />
===Plato method===<br />
<br />
[[Image:Plato immersion.png|left|frame|Idealized illustration of the "Becke lines" in the Plato immersion technique.]]<br />
<br />
Like the Anderson immersion method, the Plato method (named after W. Plato) is an immersion technique to compare a gemstone with a liquid of known index of refraction. Both methods are derivations of the Becke line method that is most often used by mineralogists.<br /><br />
As with the Becke line technique, the Plato method requires magnification with a microscope.<br />
<br />
A stone is immersed in a liquid of know index of refraction and the immersion dish is placed on the microscope stage with transmitted light. The diaphragm is closed to approximate the diameter of the stone and the focus of the lenses is just above the stone. When the facets edges of the stone appear dark, the index of refraction of the liquid is higher than that of the stone. On focusing inside the stone, the facet edges start to appear bright.<br />
<br />
When the reverse happens (white on focus above the stone and dark when the focus is lowered into the stone), the stone has a lower index of refraction compared to the liquid. When the RI of the liquid is equal (or almost equal) to the RI of the stone, the edges will remain bright.<br />
<br />
Although this technique is fairly straightforward, it does require some practice with stones and liquids of known indices of refraction. Especially trying to focus just above the stone can be cumbersome as the image of the stone is very blurry at that point. On lowering the focus inside the stone more facet edges will start to appear that can provide the information one is searching for.<br />
<br />
This technique should not be confused with the color concentration on facet edges in some diffused stones as sapphire.<br />
<br clear="all" /><br />
<br />
==References==<br />
<br />
* ''Guide to Affordable Gemology'' (2001) - Dr. W. Wm. Hanneman<br />
* ''Gemmology'' 3rd edition (2005) - Peter G. Read ISBN 0750664495<br />
* ''Gems Their Sources, Descriptions and Identification'' 4th Edition (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''Introduction to Optical mineralogy'' 2004 - William D. Nesse ISBN 0195149106</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Immersion_cell&diff=7697Immersion cell2009-02-26T11:01:26Z<p>Doos: /* Plato method */</p>
<hr />
<div>Immersion cells are use to hold liquids in which a gemstone is placed. The main objectives are to reduce reflections so one can can get a clearer view of the internal features inside the gem, or as a means to estimate the index of refraction of a gemstone through comparising.<br /><br />
Another use is to estimate the index of refraction of an inclusion in a gemstone (the gemstone itself will act as an immersion cell). <br />
__TOC__<br />
<br />
[[Image:Immersion reflection.png|left|250px|thumb|Refraction in a mineral (left) and the lack thereof when immersed in a liquid of equal index of refraction (right).]]<br />
There are circumstances when one can not use the gemological [[refractometer]] but still needs to know the approximate index of [[refraction]] of a gemstone. One method to accomplish this is to compare the gemstone with liquids of known index of refraction.<br /><br />
<br />
When light passes through a mineral the light will be refracted and reflected by the mineral, due to its higher index of refraction compared to air. This makes it possible for us to see the shape of the mineral (color disregarded). If we were to immerse the mineral in a clear liquid that has the same index of refraction as the mineral, the light will not bend inside the mineral, nor will there be any reflection. This causes the mineral to virtually disappear in the liquid.<br />
<br />
The mineral will stand out against its surrounding medium at a certain amount, something we call "relief". The larger the difference in RI between the mineral and its surrounding, the higher its relief (the more clear it will stand out). Not only air or liquids can be used as the surrounding media, a gemstone itself can be that surrounding medium for an inclusion.<br />
<br />
<br clear="all" /><br />
==Refractometry==<br />
<br />
===Anderson method===<br />
<br />
[[Image:Anderson method2.jpg|thumb|300px|right|The Anderson immersion method of comparative refraction.]]<br />
It is no secret that when you insert a drop of water in a glass filled with water, the drop will disappear. Although there are other reasons to why that happens, the focus now will be on the indices of refraction of the drop of water and the water in the glass.<br />
<br />
As the drop has the same index of refraction as the water that is already in the glass, light will travel through it at the same apparent speed. This means that the drop of water will not bend the light more (or less) than the water it is now in (the water filled glass); the drop of water will disappear like magic.<br /><br />
If we were to drop a small glass bead (RI ~ 1.5) in the water (RI ~ 1.3) then there is a difference between the two indices of refraction and you will be able to see the glass bead in the water (although not as clearly as when it was in air (RI ~ 1). The glass bead will refract the light more than the surrounding water and this can be a very useful tool to determine the index of refraction of the glass or any other transparent material like a gemstone.<br />
<br />
It should be clear by now that the closer the indices of refraction between the two media (liquid and gemstone), the more the stone will disappear when it is immersed in the liquid.<br />
<br clear="all" /><br />
<br />
[[Image:Anderson method lens.png|thumb|150px|left|Illustration on the bending of light in a lens.]]<br />
<br />
The famous gemologist B.W. Anderson devised a technique to make this visible with the aid of a series of comparative liquids. This technique is also named the "Anderson immersion method".<br /><br />
When a cut gemstone is placed table down inside a dish of water, the gem will act like a lens if the RI of the gem is larger than its environment (in this case water). Because of the difference in RI between the gemstone and the liquid (not just water) the gemstone will cast an inward shadow on the bottom of the glass. This shadow will be shown as a dark rim around the contour of the gemstone and the wider the rim, the larger the difference in indices of refraction between the stone and the liquid.<br /><br />
By placing a gemstone in a series of liquids with increasing (or decreasing) indices of refraction one can estimate the RI of the stone at hand. '''The finer the rim, the closer the RI of the gemstone matches the RI of the liquid'''.<br />
<br />
When the stone has an index of refraction lower than that of the liquid, the result is reversed and one will see a bright rim around a darker core. <br />
<br />
In order to observe this rim one only needs to insert a small mirror below the glass to redirect the image to the eye. You could of course lift the glass and observe from below, but that would require a ceiling lamp (and in time a physiotherapist).<br /><br />
As the lightsource needs to be above the immersion cell (the glass beaker) it is advisable that the light is diffused before falling on the mirror to reduce reflections from the light source. For this purpose one can use an immersion cell with a frosted bottom or create the diffusion by a thin sheet of white toilet paper (peeled).<br />
<br clear="all" /><br />
<br />
{| style="border: solid black 0px;" align="center" width="420" <br />
|-<br />
|align="center"|<br />
[[Image:Anderson rims.png|400px|thumb|From left to right: casted rims (shadows) by a brilliant cut gemstone for decreasing RI difference between liquid and gemstone. From thick to fine.]]<br />
|}<br />
<br />
The thickness of the rim should always be judged in relation to the size of the gemstone. In other words the shadow is a percentage of the diameter of the gemstone.<br /><br />
When the immersion liquid is water it is obvious that a diamond will show a relative thick rim, while a sapphire will cast a lesser thick one. If you put two of these stones next to eachother in an immersion cell (a small diamond and a large sapphire), odds are that the absolute thickness of the casted shadows are the same or similar. For that reason one needs to estimate the rims in relation to the diameter of the stones.<br />
<br />
There are many liquids one can use and in cases that this technique needs to be applied to estimate the refractive index of a gemstone, it is best to immerse the stone in several liquids with progressive RI values. The RI of the stone will lie between the RI values of the liquids in which the stone casts the smallest shadows. In cases where there is no casted shadow, the RI of the liquid is equal to the RI of the gem (like the drop of water in a glass of water).<br />
<br />
===Plato method===<br />
<br />
[[Image:Plato immersion.png|left|frame|Idealized illustration of the "Becke lines" in the Plato immersion technique.]]<br />
<br />
Like the Anderson immersion method, the Plato method (named after W. Plato) is an immersion technique to compare a gemstone with a liquid of known index of refraction. Both methods are derivations of the Becke line method that is most often used by mineralogists.<br /><br />
As with the Becke line technique, the Plato method requires magnification with a microscope.<br />
<br />
A stone is immersed in a liquid of know index of refraction and the immersion dish is placed on the microscope stage with transmitted light. The diaphragm is closed to approximate the diameter of the stone and the focus of the lenses is just above the stone. When the facets edges of the stone appear dark, the index of refraction of the liquid is higher than that of the stone. On focusing inside the stone, the facet edges start to appear bright.<br />
<br />
When the reverse happens (white on focus above the stone and dark when the focus is lowered into the stone), the stone has a lower index of refraction compared to the liquid. When the RI of the liquid is equal (or almost equal) to the RI of the stone, the edges will remain bright.<br />
<br />
Although this technique is fairly straightforward, it does require some practice with stones and liquids of known indices of refraction. Especially trying to focus just above the stone can be cumbersome as the image of the stone is very blurry at that point. On lowering the focus inside the stone more facet edges will start to appear that can provide the information one is searching for.<br />
<br />
This technique should not be confused with the color concentration on facet edges in some diffused stones as sapphire.<br />
<br clear="all" /><br />
<br />
==References==<br />
<br />
* ''Guide to Affordable Gemology'' (2001) - Dr. W. Wm. Hanneman<br />
* ''Gemmology'' 3rd edition (2005) - Peter G. Read ISBN 0750664495<br />
* ''Gems Their Sources, Descriptions and Identification'' 4th Edition (1990) - Robert Webster ISBN 0750658568 (6th ed.)</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Quahog&diff=7696Quahog2009-02-19T12:31:33Z<p>Doos: </p>
<hr />
<div>Also known as “The Northern Quahog” or “Hard Clam,” this marine bivalve is found along the eastern seaboard of the United States from Maine to Florida, and is especially common from Massachusetts to New Jersey<br />
<br />
Its name is generally pronounced as KO-hog, although “kwag” or “KWA-hog” is also used in some regions of Rhode Island, USA, and derives from the Narragansett Indians’ name for the clam, "poquauhock.” The interior lip of the clam’s shell is commonly colored light to royal purple to dark purple in stark contrast to the remainder of the mostly-white shell. This segment of the shell was ground into purple beads by Native Americans living along the eastern seaboard of the United States to form “wampum,” an important item of barter among Native Americans. Hence this species of bivalve carries the scientific name of ''Mercenaria mercenaria'', derived from the Latin word for “money”. (Older references may cite this bivalve as ''Venus mercenaria'', but it was reassigned by biologists.)<br />
<br />
=== Quahog Pearls ===<br />
<br />
The Quahog sometimes produces concretions of shell material which range in shape from flattened disks (aptly described as “shaped like an M&M candy”) to spheres, and in color from white to pale purple to a deep purple-black. '''These concretions lack the lustrous nacreous layers that are characteristic of gem pearls''' derived from oysters and are composed of layers of aragonite crystals interspersed with organic material.</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Amethyst&diff=7695Amethyst2009-02-19T12:29:39Z<p>Doos: /* Magnification */</p>
<hr />
<div>{{amethyst}}<br />
[[Image:Amethyst.jpg|left|framed|Faceted Amethyst <br /> Photo courtesy of <br />Precision Gems]]<br clear="left" /><br />
{{images}}<br />
<br />
Amethyst is a purple variety of the mineral [[quartz]]. It occurs in all intensities of the color purple from a light pastel to a depth of royal splendor. Until the beginning of the 20th century it was quite rare and costly. When vast deposits were found in Brazil, amethyst became very accessible and affordable. Amethyst has always been linked to the thinking process, ensuring clarity of vision. It inspires creativity, courage and valor.<br />
Amethyst has been successfully synthesized in the lab, so buyers need to be sure their source is qualified to separate natural from lab grown material.<br />
===Magnification===<br />
<br />
Amethyst is a type I stone in the GIA clarity grading system is usually free from eye visible inclusions.<br />
Typical inclusions visible with magnification are:<br />
* liquid feathers "fingerprints" (also known as "tiger stripes" or zebra stripes")<br />
* 2-phase, 3-phase inclusions<br />
* negative crystals<br />
<br />
[[Image:amethyst_fingerprint.jpg|left|thumb|240px|classic "fingerprint" inclusion <br />30X Magnification <br />By Barbra Voltaire]]<br />
<br /><br />
<br /><br />
<br /><br />
<br /></div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=User:Doos&diff=7694User:Doos2009-01-09T14:29:32Z<p>Doos: /* Webmaster */</p>
<hr />
<div>==Location==<br />
<br />
The Netherlands<br />
<br />
==Education==<br />
<br />
* MTS Vakschool Schoonhoven - Goldsmithing<br />
* MTS Vakschool Schoonhoven - Silversmithing<br />
* MTS Vakschool Schoonhoven - Decorating techniques (Mokume gane/Aluminum/Titanium)<br />
* MTS Vakschool Schoonhoven - Jeweler<br />
* Dutch Federation Gold and Silver - Appraiser<br />
* Insurance consultant/adjuster<br />
* Gem-A - Graduate gemmologist (FGA)<br />
<br />
==Online experience==<br />
<br />
===Webmaster===<br />
<br />
* [http://yey.be/ Yey.be]<br />
* Internal Reflections (discontinued)<br />
* <span class="plainlinks">[http://gemologyproject.com The Gemology Project]</span><br />
* [http://parels-ael.nl Parels-AEL] (in Dutch)<br />
* [http://edelsteen.nl Edelsteen.nl] (in Dutch)<br />
* [http://antiquejewelryuniversity.org Antique Jewelry University]<br />
<br />
===Consultancies/Programming===<br />
<br />
* [http://gemologyonline.com The GemologyOnline Forum]<br />
* [http://gemologytools.com Gemology Tools]<br />
* EScreeningRoom (discontinued)<br />
<br />
===Authoring===<br />
<br />
* [http://langantiques.com Lang Antiques]<br />
<br />
===Moderation===<br />
<br />
* [http://www.925-1000.com/forum/index.php The 925-1000.com Silver Marks Forum]</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=User:Doos&diff=7693User:Doos2009-01-09T14:29:20Z<p>Doos: /* Webmaster */</p>
<hr />
<div>==Location==<br />
<br />
The Netherlands<br />
<br />
==Education==<br />
<br />
* MTS Vakschool Schoonhoven - Goldsmithing<br />
* MTS Vakschool Schoonhoven - Silversmithing<br />
* MTS Vakschool Schoonhoven - Decorating techniques (Mokume gane/Aluminum/Titanium)<br />
* MTS Vakschool Schoonhoven - Jeweler<br />
* Dutch Federation Gold and Silver - Appraiser<br />
* Insurance consultant/adjuster<br />
* Gem-A - Graduate gemmologist (FGA)<br />
<br />
==Online experience==<br />
<br />
===Webmaster===<br />
<br />
* [http://yey.be/ Yey.be]<br />
* Internal Reflections (dicontinued)<br />
* <span class="plainlinks">[http://gemologyproject.com The Gemology Project]</span><br />
* [http://parels-ael.nl Parels-AEL] (in Dutch)<br />
* [http://edelsteen.nl Edelsteen.nl] (in Dutch)<br />
* [http://antiquejewelryuniversity.org Antique Jewelry University]<br />
<br />
===Consultancies/Programming===<br />
<br />
* [http://gemologyonline.com The GemologyOnline Forum]<br />
* [http://gemologytools.com Gemology Tools]<br />
* EScreeningRoom (discontinued)<br />
<br />
===Authoring===<br />
<br />
* [http://langantiques.com Lang Antiques]<br />
<br />
===Moderation===<br />
<br />
* [http://www.925-1000.com/forum/index.php The 925-1000.com Silver Marks Forum]</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=User:Doos&diff=7692User:Doos2009-01-09T14:28:52Z<p>Doos: /* Webmaster */</p>
<hr />
<div>==Location==<br />
<br />
The Netherlands<br />
<br />
==Education==<br />
<br />
* MTS Vakschool Schoonhoven - Goldsmithing<br />
* MTS Vakschool Schoonhoven - Silversmithing<br />
* MTS Vakschool Schoonhoven - Decorating techniques (Mokume gane/Aluminum/Titanium)<br />
* MTS Vakschool Schoonhoven - Jeweler<br />
* Dutch Federation Gold and Silver - Appraiser<br />
* Insurance consultant/adjuster<br />
* Gem-A - Graduate gemmologist (FGA)<br />
<br />
==Online experience==<br />
<br />
===Webmaster===<br />
<br />
* [http://yey.be/ Yey.be]<br />
* [http://internalreflections.com Internal Reflections] (dicontinued)<br />
* <span class="plainlinks">[http://gemologyproject.com The Gemology Project]</span><br />
* [http://parels-ael.nl Parels-AEL] (in Dutch)<br />
* [http://edelsteen.nl Edelsteen.nl] (in Dutch)<br />
* [http://antiquejewelryuniversity.org Antique Jewelry University]<br />
<br />
===Consultancies/Programming===<br />
<br />
* [http://gemologyonline.com The GemologyOnline Forum]<br />
* [http://gemologytools.com Gemology Tools]<br />
* EScreeningRoom (discontinued)<br />
<br />
===Authoring===<br />
<br />
* [http://langantiques.com Lang Antiques]<br />
<br />
===Moderation===<br />
<br />
* [http://www.925-1000.com/forum/index.php The 925-1000.com Silver Marks Forum]</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Kyanite&diff=7691Kyanite2009-01-06T14:39:12Z<p>Doos: /* Color */</p>
<hr />
<div>{{Kyanite}}<br />
[[Image:Kyanite.gif|left|thumb|250px|Faceted Kyanite <br /> Photo courtesy of The Gem Trader]] <br clear="left" /><br />
<br />
Kyanite is an aluminiumsilicate with the chemical formula Al<sub>2</sub>SiO<sub>5</sub>. Its name derives from the Greek word "kyanos" wich means blue. <br /><br />
The colour is blue to colourless, blue-green and brown with vitreous lustre. <br />
<br />
Kyanite together with andalusite and silimanite, all gemstones, belongs to the same polymorphic family. All are isolated tetrahedral silicates and have the same chemical formula but have distinctly different structures.<br />
<br />
Kyanite is a metamorphic mineral that occours in schists, gneisses and granite pegamatites. Associated minerals are quartz, feldspar, mica, garnet, corundum and staurolite.<br />
<br />
Kyanite occurs as bladed and tabular triclinic crystals. Lamellar twinning is common. It has two cleavage directions, one perfect and the other one good-uneven. It has directional hardness with 4 in the direction of the c-axis and 7.5 in right angles to the c-axis. <br />
<br />
Localities: Brazil, Kenya, Mocambique, Norway, Myanmar, Austria, Switzerland etc.<br />
<br />
Synonyms: Cyanite, Disthene.<br />
<br />
==Diagnostics==<br />
<br />
Kyanite may be confused with:<br />
* [[Sapphire]]<br />
* [[Spinel]]<br />
* [[Tanzanite]]<br />
* [[Idocrase]]<br />
<br />
===Diaphaneity===<br />
<br />
Transparent to translucent.<br />
<br />
===Color===<br />
<br />
Kyanite is allochromatic and occurs in the colors blue to colorless, blue-green, brown and orange.<br /><br />
The blue variety is the most used as a gemstone.<br />
<br />
The cause of color is iron and titanium for blue stones (charge transfer from Fe<sup>2+</sup> --> Ti<sup>4+</sup>) and vanadium for green ones. Orange stones are probably colored by iron and/or manganese.<br />
<br />
===Hardness===<br />
<br />
Kyanite has directional hardness with 4 to 5.5 in the direction of the c-axis and 7 to 7.5 at right angles to the c-axis. <br />
<br />
===Cleavage===<br />
<br />
Kyantite has perfect cleavage along one pism direction {100} and good cleavage along the {010} plane. It also has basal parting {001}.<br />
<br />
===Streak===<br />
<br />
White.<br />
<br />
===Refractometer===<br />
<br />
n<sub>α</sub> = 1.710 - 1.718, n<sub>β</sub> = 1.719 - 1.724, n<sub>γ</sub> = 1.724 - 1.734 with a birefringence of 0.012 to 0.017.<br /><br />
Optical nature: biaxial negative.<br />
<br />
===Pleochroism===<br />
<br />
Moderate to strong (weak in orange stones).<br /><br />
Blue stones: colorless, blue, darkblue.<br />
<br />
===Luminescence===<br />
<br />
LW-UV: weak red.<br />
<br />
===Spectroscope===<br />
<br />
[[Image:Kyanite spectrum.jpg|left|thumb|300px|Spectrum of green and some blue kyanite]]<br />
<br />
Blue kyanite may show two lines in the blue with a general cut-off in the violet. Other lines in the red and deep red may be seen in bluish green kyanite.<br /><br />
Absorption lines: (706), (689), (671), (652), 445, 435.<br />
<br />
Notice that the image resembles the "450 complex" of iron rich [[sapphire]]. In this image the 445 and 435 nm lines are shown aswell as the cut-off in the violet.<br />
<br />
<br clear="all" /><br />
<br />
For orange stones there can be a line at 553 and a general absorption in the blue-green/blue.<br />
<br />
===Specific Gravity===<br />
<br />
Kyanite can have a specific gravity from 3.53 to 3.68, but for gem material it is usually in the higher 3.67 region. It sinks in all common heavy liquids.<br />
<br />
===Magnification===<br />
<br />
* Strong colorzoning<br />
* Parallel needles<br />
* Liquid feathers<br />
<br />
==Durability==<br />
<br />
Due to its perfect to good cleavage along the {100} and {010} prism planes, the stone should be protected from being knocked in the direction of the prism faces. It also shows good parting along the basal {001} plane.<br /><br />
In addition the relative hardness of 4-5.5 in the direction of the prism faces makes it an even less candidate to be set in jewelry pieces that are prone to abbrasion, as rings.<br />
<br />
==Phenomena==<br />
<br />
Chatoyancy is reported, but rare.<br />
<br />
==Sources==<br />
<br />
* ''Gems sixth edition'' (2006) - Michael O'Donoghue ISBN 0750658568<br />
* ''Gemstones of the world, 3rd Rev Exp edition'' (2006) - Walter Schuman ISBN 1402740166<br />
* ''Mineralogy second edition'' (2002) - Dexter Perkins ISBN 0130620998<br />
* ''Gem Reference Guide'' (1995) - GIA ISBN 0873110196<br />
* ''[http://www.geminterest.com/articlist.php FlashData #25: Kyanite orange Tanzanie] - J.-M. Arlabosse, 2008</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Kyanite&diff=7690Kyanite2008-12-24T11:58:09Z<p>Doos: </p>
<hr />
<div>{{Kyanite}}<br />
[[Image:Kyanite.gif|left|thumb|250px|Faceted Kyanite <br /> Photo courtesy of The Gem Trader]] <br clear="left" /><br />
<br />
Kyanite is an aluminiumsilicate with the chemical formula Al<sub>2</sub>SiO<sub>5</sub>. Its name derives from the Greek word "kyanos" wich means blue. <br /><br />
The colour is blue to colourless, blue-green and brown with vitreous lustre. <br />
<br />
Kyanite together with andalusite and silimanite, all gemstones, belongs to the same polymorphic family. All are isolated tetrahedral silicates and have the same chemical formula but have distinctly different structures.<br />
<br />
Kyanite is a metamorphic mineral that occours in schists, gneisses and granite pegamatites. Associated minerals are quartz, feldspar, mica, garnet, corundum and staurolite.<br />
<br />
Kyanite occurs as bladed and tabular triclinic crystals. Lamellar twinning is common. It has two cleavage directions, one perfect and the other one good-uneven. It has directional hardness with 4 in the direction of the c-axis and 7.5 in right angles to the c-axis. <br />
<br />
Localities: Brazil, Kenya, Mocambique, Norway, Myanmar, Austria, Switzerland etc.<br />
<br />
Synonyms: Cyanite, Disthene.<br />
<br />
==Diagnostics==<br />
<br />
Kyanite may be confused with:<br />
* [[Sapphire]]<br />
* [[Spinel]]<br />
* [[Tanzanite]]<br />
* [[Idocrase]]<br />
<br />
===Diaphaneity===<br />
<br />
Transparent to translucent.<br />
<br />
===Color===<br />
<br />
Kyanite is allochromatic and occurs in the colors blue to colorless, blue-green, brown and orange.<br /><br />
The blue variety is the most used as a gemstone.<br />
<br />
The cause of color is iron and titanium for blue stones (charge transfer from Fe<sup>2+</sup> --> Ti<sup>4+</sup>) and vanadium for green ones. Orange stones are probably colored by manganese.<br />
<br />
===Hardness===<br />
<br />
Kyanite has directional hardness with 4 to 5.5 in the direction of the c-axis and 7 to 7.5 at right angles to the c-axis. <br />
<br />
===Cleavage===<br />
<br />
Kyantite has perfect cleavage along one pism direction {100} and good cleavage along the {010} plane. It also has basal parting {001}.<br />
<br />
===Streak===<br />
<br />
White.<br />
<br />
===Refractometer===<br />
<br />
n<sub>α</sub> = 1.710 - 1.718, n<sub>β</sub> = 1.719 - 1.724, n<sub>γ</sub> = 1.724 - 1.734 with a birefringence of 0.012 to 0.017.<br /><br />
Optical nature: biaxial negative.<br />
<br />
===Pleochroism===<br />
<br />
Moderate to strong (weak in orange stones).<br /><br />
Blue stones: colorless, blue, darkblue.<br />
<br />
===Luminescence===<br />
<br />
LW-UV: weak red.<br />
<br />
===Spectroscope===<br />
<br />
[[Image:Kyanite spectrum.jpg|left|thumb|300px|Spectrum of green and some blue kyanite]]<br />
<br />
Blue kyanite may show two lines in the blue with a general cut-off in the violet. Other lines in the red and deep red may be seen in bluish green kyanite.<br /><br />
Absorption lines: (706), (689), (671), (652), 445, 435.<br />
<br />
Notice that the image resembles the "450 complex" of iron rich [[sapphire]]. In this image the 445 and 435 nm lines are shown aswell as the cut-off in the violet.<br />
<br />
<br clear="all" /><br />
<br />
For orange stones there can be a line at 553 and a general absorption in the blue-green/blue.<br />
<br />
===Specific Gravity===<br />
<br />
Kyanite can have a specific gravity from 3.53 to 3.68, but for gem material it is usually in the higher 3.67 region. It sinks in all common heavy liquids.<br />
<br />
===Magnification===<br />
<br />
* Strong colorzoning<br />
* Parallel needles<br />
* Liquid feathers<br />
<br />
==Durability==<br />
<br />
Due to its perfect to good cleavage along the {100} and {010} prism planes, the stone should be protected from being knocked in the direction of the prism faces. It also shows good parting along the basal {001} plane.<br /><br />
In addition the relative hardness of 4-5.5 in the direction of the prism faces makes it an even less candidate to be set in jewelry pieces that are prone to abbrasion, as rings.<br />
<br />
==Phenomena==<br />
<br />
Chatoyancy is reported, but rare.<br />
<br />
==Sources==<br />
<br />
* ''Gems sixth edition'' (2006) - Michael O'Donoghue ISBN 0750658568<br />
* ''Gemstones of the world, 3rd Rev Exp edition'' (2006) - Walter Schuman ISBN 1402740166<br />
* ''Mineralogy second edition'' (2002) - Dexter Perkins ISBN 0130620998<br />
* ''Gem Reference Guide'' (1995) - GIA ISBN 0873110196<br />
* ''[http://www.geminterest.com/articlist.php FlashData #25: Kyanite orange Tanzanie] - J.-M. Arlabosse, 2008</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Jasper&diff=7689Jasper2008-12-06T11:56:14Z<p>Doos: /* Scenic jasper */</p>
<hr />
<div>{{Jasper}}<br />
[[Image:Imperial_Jasper,_Mexico.JPG|left|framed|Imperial Jasper Cabochon, Mexico <br /> Photo courtesy of Rick Martin]]<br clear="left" /><br />
<br />
==Scenic jasper==<br />
<br />
[[Image:Scenic jasper.jpg|left|thumb|200px|Picture jasper, silhouette of a young woman<br />Image courtesy of Topsieraden.nl]]<br />
<br />
Picture, or scenic, or landscape jasper and agate gets its name from the pictures of scenes, animals or objects formed by the colorful patterns from other minerals present.</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Jasper&diff=7688Jasper2008-12-06T11:55:07Z<p>Doos: added contribution (id:427)</p>
<hr />
<div>{{Jasper}}<br />
[[Image:Imperial_Jasper,_Mexico.JPG|left|framed|Imperial Jasper Cabochon, Mexico <br /> Photo courtesy of Rick Martin]]<br clear="left" /><br />
<br />
==Scenic jasper==<br />
<br />
[[Image:Scenic jasper.jpg|left|thumb|200px|Scenic jasper.<br />Image courtesy of Topsieraden.nl]]<br />
<br />
Picture, or scenic, or landscape jasper and agate gets its name from the pictures of scenes, animals or objects formed by the colorful patterns from other minerals present.</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=File:Scenic_jasper.jpg&diff=7687File:Scenic jasper.jpg2008-12-06T11:53:16Z<p>Doos: Scenic jasper.
Image courtesy of topsieraden.nl</p>
<hr />
<div>Scenic jasper.<br />
Image courtesy of topsieraden.nl</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Jadeite&diff=7686Jadeite2008-12-05T12:34:32Z<p>Doos: Addition from Robert Biehl (id:432)</p>
<hr />
<div>{{jadeite}}<br />
<br />
Jadeite is made up of interlocking pyroxene crystals. It occurs in a vary wide range of colors like green, lilac, white, pink, brown, red, blue, black, orange and yellow. The most prized color is a rich emerald green and is called Imperial Jade. Its green color is due to its chromium content and can be distinguished with a Chelsea (jadeite) filter. Jadeite is believed to prevent/cure hip and kidney ailments. <br />
<br />
==Enhancements==<br />
<br />
Common enhancements to jadeite:<br />
<br />
* Fracture filling - wax - conceal cracks and fractures<br />
* Coatings - wax - to improve luster<br />
* Staining - color improvement through dyes<br />
* Bleaching - removes stains <br />
* Polymer impregnation - improves luster and to stabilize piece after bleaching<br />
<br />
==Occurrence==<br />
The most important source of jadeite is Myanmar but Guatemala, Japan and the USA (California) are also important sources<br />
<br />
==Sources consulted==<br />
*Smithsonian Handbooks, Gemstones, Second Edition 2002</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Nephrite&diff=7685Nephrite2008-12-05T12:32:22Z<p>Doos: addition from Robert Biehl (id:431)</p>
<hr />
<div>{{nephrite}}<br />
<br />
Nephrite has been recognized as a separate type of jade since 1863. It is formed from aggregates of fibrous amphibole crystals. The structure they form is interlocking and tougher than steel. It's colors range from dark green iron rich varieties to cream colored magnesium rich varieties. It can be found blotchy, banded or singly colored. It is vary popular for carving and was used for weapons of the past. <br />
<br />
==Enhancements==<br />
<br />
Common enhancements to nephrite:<br />
<br />
* Fracture filling - wax - conceal cracks and fractures<br />
* Coatings - wax - to improve luster<br />
<br />
==Occurrence==<br />
Nephrite is found in Turkestan, Myanmar, Siberia (dark green rocks with black spots), Russia, China, New Zealand, Australia (black stones), USA, Canada, Mexico, Brazil, Taiwan, Zimbabwe (dark green), Italy, Poland, Germany and Switzerland. It has been carved by the Chinese for at least 2,000 years. <br />
<br />
==Sources consulted==<br />
*Smithsonian Handbooks, Gemstones, Second Edition 2002</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Specific_Gravity&diff=7684Specific Gravity2008-12-01T10:56:01Z<p>Doos: </p>
<hr />
<div>The specific gravity (SG) of gemstones is a constant widely used in gemological property charts. Although not every gemologist enjoys doing an SG test, it is still a property which can be very useful when other general tests fail.<br /><br />
The method of determination uses a [[Hydrostatic Balance|hydrostatic balance]].<br />
<br />
==Basic==<br />
<br />
Specific gravity (also known as "relative density") is the ratio between the weight of a stone in air and the weight of an equal volume in water. By convention, the temperature of the water is 4° C and at standard atmosphere because the density of water is greatest under these conditions. Room temperature conditions are adequate for gemological purposes, as the small difference in density of the water will have little effect on the readings (measured to the second decimal).<br />
<br />
Since specific gravity is relative to the weight of an object in air and its weight in water, it is a ratio and isn't expressed in units (such as kg/m³). For instance, the SG of Diamond = 3.52 (whereas the density of Diamond = 3.52 g/cm³).<br />
<br />
In gemology, specific gravity is, usually, determined through an apparatus based on Archimedes' Principle.<br /><br />
Archimedes' Principle (or the Law of Buoyancy) states that: the upward force on an immersed object is equal to the weight of the displaced fluid.<br /><br />
This may sound complicated but it is a fairly simple, yet brilliant, law.<br />
<br />
Consider two balls of equal weight but of different specific gravity, for instance a 10 gram gold ball and a 10 gram silver ball. The gold ball has a sg of 19.3, while silver has a sg of 10.5.<br /><br />
Because gold has a sg that is almost twice as much as silver, you can imagine that the 10 gram gold ball will be smaller than the 10 gram silver ball. In other words, the gold ball will have a smaller volume than the silver ball.<br /><br />
When you hang both balls in water (immersed), then the silver ball will displace much more water than the golden ball due to its higher volume.<br />
<br />
Water has a sg of 1, so the weight of a cubic centimeter of water is 1 gram (actually 0.0098 Newton, but grams used for simplicity). Through some simple math we can calulate the volume of the balls. The volume of the gold ball is 0.52 cubic centimeter and the volume of the silver ball is 0.95 cubic centimeter (volume is mass divided by density).<br /><br />
From this we can conclude that the silver ball will displace 0.95 cubic centimeter of water, which weighs 0.95 gram. The golden ball will displace 0.52 grams of water (because 1 cubic centimeter of water weighs 1 gram).<br />
<br />
Now back to Archimedes' Principle: the upward force on an immersed object is equal to the weight of the displaced fluid (the fluid being water in this case). The silver ball displaces a higher weight of water, so it will experience a larger upward force than the golden ball and will rise higher in the water when immersed.<br />
<br />
A common mistake is to drop the object in the water in a way that it will sink to the bottom. It can not work in that case as it is then no longer "immersed".<br />
<br />
===Density===<br />
<br />
Density is different from specific gravity in that it is the mass of an object divided by its volume, expressed in kg/m³ by SI (Le Système International d'Unités - The International System of Units) standards. In gemology, g/cm³ is used. Other weighing systems are still widely in use (mostly in the USA and the UK), but the metric system of the SI is slowly finding its way there as well.<br />
<br />
===Mass and weight===<br />
<br />
Mass is the amount of material in an object and is a physical property of that object (like a gemstone), expressed in kg (kilogram) by SI standards.<br />
<br />
Weight is the gravitational force (9.8 m/s²) on that object and is expressed in N (newton). Weight is not a physical property as it may change under different situations. A stone would weigh less on Earth's moon than on Earth, but its mass would remain the same.<br />
<br />
As can be concluded, we should use "mass" instead of "weight" when being scientifically correct, but in daily use mass and weight are interchangeable.<br />
<br />
The carat (ct) is an accepted unit of mass (or weight, if you please).<br />
<br />
===Measurement of specific gravity===<br />
<br />
The method of measuring SG is with a hydrostatic balance.<br /><br />
First, the stone is weighed in air and then weighed when fully immersed in water. After this, the weights are inserted into a simple formula.<br />
<br />
:<math> SG = \frac{weight\ of\ stone\ in\ air}{weight\ of\ stone\ in\ air\ -\ weight\ of\ stone\ in\ water}</math><br />
<br />
<br />
A demonstration can be seen in this video.<br />
<br />
====Video presentation====<br />
<br />
{| {{table}}<br />
|-<br />
|[[image:video.png]] [http://video.google.com/videoplay?docid=4628022311595565279 Specific Gravity Video] (Hosted by Google Video)<br />
|-<br />
|Video showing the method of determining hydrostatic specific gravity - WMV/video format - 7.96MB<br />
|}<br />
<br />
==Related topics==<br />
<br />
* [[Hydrostatic Balance]]</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Spinel&diff=7683Spinel2008-09-26T11:07:21Z<p>Doos: /* Sources */</p>
<hr />
<div>{{spinel}}<br />
[[Image:Badak1.JPG|left|framed|Badakshan spinel, Afghanistan]] <br clear="left" /><br />
{{images}}<br />
<br />
Spinel is a mineral group. For many centuries, most gem spinels were misidentified as [[sapphire]] or [[ruby]] because they have similar properties and occur in the same geological deposits. The historically significant 5.08 centimeter "Black Prince Ruby" in the center of the British Imperial Crown was only recently identified as a spinel. This stone is irregular in shape and has a somewhat squareish outline. Additionally, it was not faceted, merely polished. Spinels also occur in a vast array of colors. They are slightly softer than sapphires but still very durable.<br>The earliest known use of spinels was as ornaments found in Buddhist tombs in Afghanistan. Blue spinels have been found in England, dating back to the Roman occupation (51 BC to 400 AD).<br />
<br />
<!-- [[Image:DSC_7858_d.jpg|left|thumb|250px|Cobalt Blue Spinel, 3.96 ct <br /> Photo by Jeff Scovil<br />Courtesy of R.W. Wise Goldsmiths]] --><br />
<br clear="all" /> <br />
<br />
==Chemical composition==<br />
<br />
Common spinel belongs to the "spinel series", which belongs to the "spinel group".<br /><br />
The general formula for the spinel group is A<sup>2+</sup>B<sup>3+</sup><sub>2</sub>O<sub>4</sub>. The 3 series of the spinel group are defined by the B<sup>3+</sup> cation.<br /><br />
The spinel group is made up of 3 [[isomorphous replacement|isomorphous]] series.<br />
<br />
The isomorphous series:<br />
* Spinel series (aluminum)<br />
** Spinel - MgAl<sub>2</sub>O<sub>4</sub> (n = 1.719, sg ~ 3.60)<br />
** Hercynite - FeAl<sub>2</sub>O<sub>4</sub><br />
** Gahnite - ZnAl<sub>2</sub>O<sub>4</sub> (n = 1.805, sg = 4.62)<br />
** Galaxite - MnAl<sub>2</sub>O<sub>4</sub><br />
<br />
* Magnetite series (ferric iron)<br />
** Magnetite - FeFe<sub>2</sub>O<sub>4</sub><br />
** Magnesioferrite - MgFe<sub>2</sub>O<sub>4</sub><br />
** Ulvöspinel - FeFeTiO<sub>4</sub><br />
** Franklinite - ZnFe<sub>2</sub>O<sub>4</sub><br />
** Jacobsite - MnFe<sub>2</sub>O<sub>4</sub><br />
** Trevorite - NiFe<sub>2</sub>O<sub>4</sub><br />
<br />
* Chromite series (chrome)<br />
** Chromite - FeCr<sub>2</sub>O<sub>4</sub><br />
** Magnesiochromite - MgCr<sub>2</sub>O<sub>4</sub><br />
<br />
Most of the above series members are rare in nature with the exception of the members of the spinel series, magnetite and chromite. To gemologists common spinel and gahnite are of most interest.<br />
<br />
When gemologist refer to "spinel", we usually imply common spinel, that is the spinel that belongs to the spinel series of the spinel group.<br />
<br />
==Diagnostics==<br />
<br />
Spinel can be confused with many stones by appearance alone, yet optical properties usually rule out most of them.<br /><br />
As spinel belongs to an isomorhous series, the optical and physical properties may vary.<br />
<br />
===Color===<br />
<br />
Spinel: colorless, green, blue, red, black.<br /><br />
Gahnite: blue-green, yellow, brown.<br />
<br />
Varieties:<br />
*Pleonaste (also named ceylonite) - (Mg,Fe)Al<sub>2</sub>0<sub>4</sub> - dark green to blue-green; black<br />
*Gahnospinel - (Mg,Zn)Al<sub>2</sub>0<sub>4</sub> - pale to dark blue, black.<br />
<br />
Spinel is allochromatic and variety colors are produced by transition metals:<br />
* Cr<sup>3+</sup> - red, pink<br />
* Fe<sup>2+</sup> - blue, violet<br />
* Fe<sup>2+</sup> + Co<sup>2+</sup> - darkblue<br />
* Fe<sup>3+</sup> - green<br />
<br />
===Diaphaneity===<br />
<br />
Transparent to opaque.<br />
<br />
===Refractometer===<br />
<br />
Spinel is isotropic and the refractive index of common spinel is generally around 1.712 to 1.720. Red spinel can have a refractive index up to 1.74.<br /><br />
The only other isotropic gemstone that falls within this range is grossular garnet, but it will usually be higher and the color is also different.<br /><br />
Other members of the spinel series, such as gahnite will have higher refractive indices.<br />
<br />
All other gemstones can easily be seperated from spinel by their optic nature.<br />
<br />
Synthetic spinel has a usual refractive index of 1.727.<br /><br />
Gahnospinels have a refractive index between 1.725 and 1.753, while pleonaste (ceylonite) has an R.I. range of 1.77 to 1.80.<br /><br />
Black spinel-hercynite (pleonaste) stones from Thailand may have an RI upto 1.789 with an average SG of 3.86 (Seriwat, 2008)<br />
<br />
===Specific gravity===<br />
<br />
As with the refractive indices, the specif gravity of spinel can vary due to isomorphous replacement.<br /><br />
The values for most gem grade material lies between 3.58 and 3.61. Pleonaste has a S.G. between 3.63 and 3.90.<br /><br />
Gahnospinels may have a specific gravity up to 4.06.<br />
<br />
===Polariscope===<br />
<br />
Common spinel is isotrope and will remain dark under crossed polars.<br /><br />
Verneuil type synthetic spinel will always (maybe with the exception of red) show strong anomalous birefringence due to excess Al<sub>2</sub>O<sub>3</sub> (see [[spinel#synthetics|synthetics]]). This anomalous extinction (as it is currently named) can be seen as "tabby" extinction, resembling the color distribution of a cat's fur, or/and as an Andreas cross caused by pseudo-birefringence.<br />
<br />
Natural and flux-melt synthetic spinel may show weak anomalous extinction.<br />
<br />
===Spectra===<br />
<br />
[[Image:Spectrum natural blue spinel iron.jpg|thumb|left|240px]]<br />Spectrum of a natural blue spinel, colored by iron and minor traces of cobalt.<br clear="all" /><br />
<br />
==Phenomena==<br />
<br />
* Asterism (4 and 6-pointed stars)<br />
* Color change (rare)<br />
<br />
==Occurrence==<br />
<br />
Myanmar; Vietnam; Thailand; Sri Lanka; Pakistan; Afghanistan; Tadshikistan; Kenia; Tanzania; South-Africa; Brazil.<br />
<br />
==Synthetics==<br />
<br />
Spinel is synthesized by the [[flame fusion|Verneuil (flame-fusion) process]] and the flux-melt method, although the first process does not render a true synthetic in most cases.<br />
<br />
===Flame fusion===<br />
<br />
Flame fusion synthetic spinels are produced since 1908 (by accident while creating synthetic corundum), but were not commercially available until 1930.<br />
<br />
It was found that while trying to synthesize spinel through the Verneuil process, the resulting boules would easily fracture and no reasonably sized gemstones could be cut from them.<br /><br />
The ratio MgO to Al<sub>2</sub>O<sub>3</sub> is 1:1 for common spinel and by changing that ratio (adding Al<sub>2</sub>O<sub>3</sub>) the boules became stable. Because that alters the chemical formula of the synthetic, it is not a true synthetic (but accepted as such). Sometimes these are named "beta-corundum" due to the excess of alumnia.<br />
<br />
For the creation of red stones, this alteration was no option and, usually, no red synthetic spinel boules created by the flame-fusion process result in large stones (but are known to excist). The larger sized red synthetic spinels are mostly created with the flux-melt method. The few red synthetics that are created through the Verneuil process will show curved striae like their synthetic corundum cousins.<br />
<br />
As a result in the changing of the MgO:Al<sub>2</sub>O<sub>3</sub> ratio, the flame fusion synthetics have higher refractive indices (usually stable at 1.727) and a higher specific gravity (3.64).<br />
<br />
{| {{table}} width="70%" style="margin-left:0;"<br />
|-<br />
|colspan="5" align="center"|Properties of flame fusion synthetic spinel<br />
|-<br />
!color<br />
!coloring agent<br />
!RI<br />
!SG<br />
!other diagnostics<br />
|-<br />
|colorless||none||1.728-1.740||3.65-3.80||LWUV: green; SWUV: white/blue<br />
|-<br />
|red||Cr<sup>3+</sup>||1.722-1.725*||3.58-3.60||curved striae; spectrum; fluorescence<br />
|- <br />
|pink||Cu||1.727-1.740||3.65-3.80<br />
|-<br />
|yellow||Mn||do.||do.||LW/SWUV: green<br />
|-<br />
|emerald green||Mn + Co<sup>3+</sup>||do.||do.<br />
|-<br />
|tourmaline green||Cr<sup>3+</sup>||do.||do.||spectrum<br />
|-<br />
|beryl green||Cr<sup>3+</sup> + Mn||do.||do.||spectrum<br />
|-<br />
|zircon blue||Co<sup>3+</sup> + Cr<sup>3+</sup> + Ti||do.||do.||spectrum<br />
|-<br />
|sapphire blue||Co<sup>3+</sup>||do.||do.||spectrum; fluorescence; CCF<br />
|-<br />
|amethyst violet||Co<sup>3+</sup> + Mn||do.||do.<br />
|-<br />
|alexandrite color change||Cr<sup>3+</sup> + V||do.||do.||spectrum<br />
|-<br />
|lapis lazuli||Co<sup>3+</sup>||1.725||3.52||spectrum; CCF<br />
|-<br />
|colspan="5"|<br />
Strong "tabby" extinction can be seen in all<br /><br />
<nowiki>*</nowiki> Henn, 1995 metions a low value of 1.720.<br />
|}<br />
<br />
===Flux-melt===<br />
<br />
First commercial production of flux-melt synthetic spinel started around 1980, although successful experiments date back to 1848 (by the French chemist Ebelmen). As of 1989 larger volumes of this synthetic appeared on the market, produced in Novosibirsk, Russia.<br />
<br />
Apart from red, blue synthetics have also been produced by the flux-melt process.<br /><br />
These are true synthetics.<br />
<br />
{| {{table}} width="70%" style="margin-left:0;"<br />
|-<br />
|colspan="5" align="center"|Properties of flux-melt synthetic spinel<br />
|-<br />
!color<br />
!coloring agent<br />
!RI<br />
!SG<br />
!other diagnostics<br />
|-<br />
|red||Cr<sup>3+</sup>||1.716-1.719||3.58-3.62||LW/SWUV: distinct red-orange/red<br />flux residu inclusions<br />
|-<br />
|blue||Co<sup>2+</sup> + Fe<sup>2+</sup>||1.719||3.58||spectrum; SWUV: inert; LWUV: weak read<br />flux residu inclusions<br />
|}<br />
Tabby extinction is also seen in these synthetics. Careful observation of inclusions is the main means of separation for red stones. Blue flux-melt synthetics can also be distinguished by the spectrum.<br />
<br />
===Czochralski pulling method===<br />
A recent development (2007) is the procuction of red to pink synthetic spinels by the Czochralski pulling method.<br />
<br />
==Inclusion images==<br />
<br />
[[image:SP713Fzm1brE2U.jpg|thumb|left|240px|Apatite inclusions and their star-like outgrowths along the 60 degree hexagonal plane - looking much like stellate dislocation systems, in a cobalt blue spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
[[image:SP820AO2-16.jpg|thumb|left|240px|Octahedral spinel inclusions with "Saturn-ring" stress fractures in a Burma red spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
[[image:SP820A02colW.jpg|thumb|left|240px|Octahedral inclusions in a Burma red spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
<br clear="all" /><br />
{{inclusions}}<br />
<br />
==Sources==<br />
* ''Introduction to Optical Mineralogy'' 2004 - William D. Nesse ISBN 0195149106<br />
* ''Gemmology 3rd edition'' (2005) - Peter G. Read ISBN 0750664495<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''Edelsteinkuntliches Praktikum'' - Ulrich Henn, Gemmologie Jahrgang 44 / Heft 4 / Dezember 1995, Spinell pp.54-62<br />
* ''Diploma course notes'' 1987 - Gem-A<br />
* ''Über die Eigenshaften von im Flussmittelverfahren hergestellten synthetischen roten und blauen Spinellen aus Russland'' - U. Henn/H. Bank, Gemmologie Jahrgang 41 / Heft 1 / April 1992, pp1-7<br />
* ''Black Opaque Gem Minerals Associated with Corundum in the Alluvial Deposits of Thailand''. Australian Gemmologist, 2008, Vol. 23, No. 6, pp. 242-253. Dr. Seriwat Saminpanya</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Fluorite&diff=7682Fluorite2008-09-20T11:59:31Z<p>Doos: /* =Optical phenomena */</p>
<hr />
<div>{{fluorite}}<br />
<br />
[[Image:fluorite.JPG|left|framed|Fluorite]]<br clear="left" /><br />
<br clear="all" /><br />
<br />
==Diagnostics==<br />
<br />
===Thermoluminescence===<br />
<br />
FLuorite may luminesce when heated. The stored energy from UV radiation is released if heated to a certain temperature and the effect depends on the amount of stored energy.<br /><br />
One can see this when a small amount of fluorite is placed on a teaspoon and heated over a candle for a few minutes (in a dark room).<br /><br />
Material from Telemark, Norway will show a blue-green thermoluminescence.<br />
<br />
===Optical phenomena===<br />
<br />
====Color change====<br />
<br />
Color change fluorite has been reported with a change from blue to purple (much like some color change garnet).</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Fluorite&diff=7681Fluorite2008-09-20T11:59:14Z<p>Doos: </p>
<hr />
<div>{{fluorite}}<br />
<br />
[[Image:fluorite.JPG|left|framed|Fluorite]]<br clear="left" /><br />
<br clear="all" /><br />
<br />
==Diagnostics==<br />
<br />
===Thermoluminescence===<br />
<br />
FLuorite may luminesce when heated. The stored energy from UV radiation is released if heated to a certain temperature and the effect depends on the amount of stored energy.<br /><br />
One can see this when a small amount of fluorite is placed on a teaspoon and heated over a candle for a few minutes (in a dark room).<br /><br />
Material from Telemark, Norway will show a blue-green thermoluminescence.<br />
<br />
===Optical phenomena==<br />
<br />
====Color change====<br />
<br />
Color change fluorite has been reported with a change from blue to purple (much like some color change garnet).</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Spinel&diff=7680Spinel2008-09-19T16:09:15Z<p>Doos: /* Sources */</p>
<hr />
<div>{{spinel}}<br />
[[Image:Badak1.JPG|left|framed|Badakshan spinel, Afghanistan]] <br clear="left" /><br />
{{images}}<br />
<br />
Spinel is a mineral group. For many centuries, most gem spinels were misidentified as [[sapphire]] or [[ruby]] because they have similar properties and occur in the same geological deposits. The historically significant 5.08 centimeter "Black Prince Ruby" in the center of the British Imperial Crown was only recently identified as a spinel. This stone is irregular in shape and has a somewhat squareish outline. Additionally, it was not faceted, merely polished. Spinels also occur in a vast array of colors. They are slightly softer than sapphires but still very durable.<br>The earliest known use of spinels was as ornaments found in Buddhist tombs in Afghanistan. Blue spinels have been found in England, dating back to the Roman occupation (51 BC to 400 AD).<br />
<br />
<!-- [[Image:DSC_7858_d.jpg|left|thumb|250px|Cobalt Blue Spinel, 3.96 ct <br /> Photo by Jeff Scovil<br />Courtesy of R.W. Wise Goldsmiths]] --><br />
<br clear="all" /> <br />
<br />
==Chemical composition==<br />
<br />
Common spinel belongs to the "spinel series", which belongs to the "spinel group".<br /><br />
The general formula for the spinel group is A<sup>2+</sup>B<sup>3+</sup><sub>2</sub>O<sub>4</sub>. The 3 series of the spinel group are defined by the B<sup>3+</sup> cation.<br /><br />
The spinel group is made up of 3 [[isomorphous replacement|isomorphous]] series.<br />
<br />
The isomorphous series:<br />
* Spinel series (aluminum)<br />
** Spinel - MgAl<sub>2</sub>O<sub>4</sub> (n = 1.719, sg ~ 3.60)<br />
** Hercynite - FeAl<sub>2</sub>O<sub>4</sub><br />
** Gahnite - ZnAl<sub>2</sub>O<sub>4</sub> (n = 1.805, sg = 4.62)<br />
** Galaxite - MnAl<sub>2</sub>O<sub>4</sub><br />
<br />
* Magnetite series (ferric iron)<br />
** Magnetite - FeFe<sub>2</sub>O<sub>4</sub><br />
** Magnesioferrite - MgFe<sub>2</sub>O<sub>4</sub><br />
** Ulvöspinel - FeFeTiO<sub>4</sub><br />
** Franklinite - ZnFe<sub>2</sub>O<sub>4</sub><br />
** Jacobsite - MnFe<sub>2</sub>O<sub>4</sub><br />
** Trevorite - NiFe<sub>2</sub>O<sub>4</sub><br />
<br />
* Chromite series (chrome)<br />
** Chromite - FeCr<sub>2</sub>O<sub>4</sub><br />
** Magnesiochromite - MgCr<sub>2</sub>O<sub>4</sub><br />
<br />
Most of the above series members are rare in nature with the exception of the members of the spinel series, magnetite and chromite. To gemologists common spinel and gahnite are of most interest.<br />
<br />
When gemologist refer to "spinel", we usually imply common spinel, that is the spinel that belongs to the spinel series of the spinel group.<br />
<br />
==Diagnostics==<br />
<br />
Spinel can be confused with many stones by appearance alone, yet optical properties usually rule out most of them.<br /><br />
As spinel belongs to an isomorhous series, the optical and physical properties may vary.<br />
<br />
===Color===<br />
<br />
Spinel: colorless, green, blue, red, black.<br /><br />
Gahnite: blue-green, yellow, brown.<br />
<br />
Varieties:<br />
*Pleonaste (also named ceylonite) - (Mg,Fe)Al<sub>2</sub>0<sub>4</sub> - dark green to blue-green; black<br />
*Gahnospinel - (Mg,Zn)Al<sub>2</sub>0<sub>4</sub> - pale to dark blue, black.<br />
<br />
Spinel is allochromatic and variety colors are produced by transition metals:<br />
* Cr<sup>3+</sup> - red, pink<br />
* Fe<sup>2+</sup> - blue, violet<br />
* Fe<sup>2+</sup> + Co<sup>2+</sup> - darkblue<br />
* Fe<sup>3+</sup> - green<br />
<br />
===Diaphaneity===<br />
<br />
Transparent to opaque.<br />
<br />
===Refractometer===<br />
<br />
Spinel is isotropic and the refractive index of common spinel is generally around 1.712 to 1.720. Red spinel can have a refractive index up to 1.74.<br /><br />
The only other isotropic gemstone that falls within this range is grossular garnet, but it will usually be higher and the color is also different.<br /><br />
Other members of the spinel series, such as gahnite will have higher refractive indices.<br />
<br />
All other gemstones can easily be seperated from spinel by their optic nature.<br />
<br />
Synthetic spinel has a usual refractive index of 1.727.<br /><br />
Gahnospinels have a refractive index between 1.725 and 1.753, while pleonaste (ceylonite) has an R.I. range of 1.77 to 1.80.<br /><br />
Black spinel-hercynite (pleonaste) stones from Thailand may have an RI upto 1.789 with an average SG of 3.86 (Seriwat, 2008)<br />
<br />
===Specific gravity===<br />
<br />
As with the refractive indices, the specif gravity of spinel can vary due to isomorphous replacement.<br /><br />
The values for most gem grade material lies between 3.58 and 3.61. Pleonaste has a S.G. between 3.63 and 3.90.<br /><br />
Gahnospinels may have a specific gravity up to 4.06.<br />
<br />
===Polariscope===<br />
<br />
Common spinel is isotrope and will remain dark under crossed polars.<br /><br />
Verneuil type synthetic spinel will always (maybe with the exception of red) show strong anomalous birefringence due to excess Al<sub>2</sub>O<sub>3</sub> (see [[spinel#synthetics|synthetics]]). This anomalous extinction (as it is currently named) can be seen as "tabby" extinction, resembling the color distribution of a cat's fur, or/and as an Andreas cross caused by pseudo-birefringence.<br />
<br />
Natural and flux-melt synthetic spinel may show weak anomalous extinction.<br />
<br />
===Spectra===<br />
<br />
[[Image:Spectrum natural blue spinel iron.jpg|thumb|left|240px]]<br />Spectrum of a natural blue spinel, colored by iron and minor traces of cobalt.<br clear="all" /><br />
<br />
==Phenomena==<br />
<br />
* Asterism (4 and 6-pointed stars)<br />
* Color change (rare)<br />
<br />
==Occurrence==<br />
<br />
Myanmar; Vietnam; Thailand; Sri Lanka; Pakistan; Afghanistan; Tadshikistan; Kenia; Tanzania; South-Africa; Brazil.<br />
<br />
==Synthetics==<br />
<br />
Spinel is synthesized by the [[flame fusion|Verneuil (flame-fusion) process]] and the flux-melt method, although the first process does not render a true synthetic in most cases.<br />
<br />
===Flame fusion===<br />
<br />
Flame fusion synthetic spinels are produced since 1908 (by accident while creating synthetic corundum), but were not commercially available until 1930.<br />
<br />
It was found that while trying to synthesize spinel through the Verneuil process, the resulting boules would easily fracture and no reasonably sized gemstones could be cut from them.<br /><br />
The ratio MgO to Al<sub>2</sub>O<sub>3</sub> is 1:1 for common spinel and by changing that ratio (adding Al<sub>2</sub>O<sub>3</sub>) the boules became stable. Because that alters the chemical formula of the synthetic, it is not a true synthetic (but accepted as such). Sometimes these are named "beta-corundum" due to the excess of alumnia.<br />
<br />
For the creation of red stones, this alteration was no option and, usually, no red synthetic spinel boules created by the flame-fusion process result in large stones (but are known to excist). The larger sized red synthetic spinels are mostly created with the flux-melt method. The few red synthetics that are created through the Verneuil process will show curved striae like their synthetic corundum cousins.<br />
<br />
As a result in the changing of the MgO:Al<sub>2</sub>O<sub>3</sub> ratio, the flame fusion synthetics have higher refractive indices (usually stable at 1.727) and a higher specific gravity (3.64).<br />
<br />
{| {{table}} width="70%" style="margin-left:0;"<br />
|-<br />
|colspan="5" align="center"|Properties of flame fusion synthetic spinel<br />
|-<br />
!color<br />
!coloring agent<br />
!RI<br />
!SG<br />
!other diagnostics<br />
|-<br />
|colorless||none||1.728-1.740||3.65-3.80||LWUV: green; SWUV: white/blue<br />
|-<br />
|red||Cr<sup>3+</sup>||1.722-1.725*||3.58-3.60||curved striae; spectrum; fluorescence<br />
|- <br />
|pink||Cu||1.727-1.740||3.65-3.80<br />
|-<br />
|yellow||Mn||do.||do.||LW/SWUV: green<br />
|-<br />
|emerald green||Mn + Co<sup>3+</sup>||do.||do.<br />
|-<br />
|tourmaline green||Cr<sup>3+</sup>||do.||do.||spectrum<br />
|-<br />
|beryl green||Cr<sup>3+</sup> + Mn||do.||do.||spectrum<br />
|-<br />
|zircon blue||Co<sup>3+</sup> + Cr<sup>3+</sup> + Ti||do.||do.||spectrum<br />
|-<br />
|sapphire blue||Co<sup>3+</sup>||do.||do.||spectrum; fluorescence; CCF<br />
|-<br />
|amethyst violet||Co<sup>3+</sup> + Mn||do.||do.<br />
|-<br />
|alexandrite color change||Cr<sup>3+</sup> + V||do.||do.||spectrum<br />
|-<br />
|lapis lazuli||Co<sup>3+</sup>||1.725||3.52||spectrum; CCF<br />
|-<br />
|colspan="5"|<br />
Strong "tabby" extinction can be seen in all<br /><br />
<nowiki>*</nowiki> Henn, 1995 metions a low value of 1.720.<br />
|}<br />
<br />
===Flux-melt===<br />
<br />
First commercial production of flux-melt synthetic spinel started around 1980, although successful experiments date back to 1848 (by the French chemist Ebelmen). As of 1989 larger volumes of this synthetic appeared on the market, produced in Novosibirsk, Russia.<br />
<br />
Apart from red, blue synthetics have also been produced by the flux-melt process.<br /><br />
These are true synthetics.<br />
<br />
{| {{table}} width="70%" style="margin-left:0;"<br />
|-<br />
|colspan="5" align="center"|Properties of flux-melt synthetic spinel<br />
|-<br />
!color<br />
!coloring agent<br />
!RI<br />
!SG<br />
!other diagnostics<br />
|-<br />
|red||Cr<sup>3+</sup>||1.716-1.719||3.58-3.62||LW/SWUV: distinct red-orange/red<br />flux residu inclusions<br />
|-<br />
|blue||Co<sup>2+</sup> + Fe<sup>2+</sup>||1.719||3.58||spectrum; SWUV: inert; LWUV: weak read<br />flux residu inclusions<br />
|}<br />
Tabby extinction is also seen in these synthetics. Careful observation of inclusions is the main means of separation for red stones. Blue flux-melt synthetics can also be distinguished by the spectrum.<br />
<br />
===Czochralski pulling method===<br />
A recent development (2007) is the procuction of red to pink synthetic spinels by the Czochralski pulling method.<br />
<br />
==Inclusion images==<br />
<br />
[[image:SP713Fzm1brE2U.jpg|thumb|left|240px|Apatite inclusions and their star-like outgrowths along the 60 degree hexagonal plane - looking much like stellate dislocation systems, in a cobalt blue spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
[[image:SP820AO2-16.jpg|thumb|left|240px|Octahedral spinel inclusions with "Saturn-ring" stress fractures in a Burma red spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
[[image:SP820A02colW.jpg|thumb|left|240px|Octahedral inclusions in a Burma red spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
<br clear="all" /><br />
{{inclusions}}<br />
<br />
==Sources==<br />
* ''Introduction to Optical Mineralogy'' 2004 - William D. Nesse ISBN 0195149106<br />
* ''Gemmology 3rd edition'' (2005) - Peter G. Read ISBN 0750664495<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''Edelsteinkuntliches Praktikum'' - Ulrich Henn, Gemmologie Jahrgang 44 / Heft 4 / Dezember 1995, Spinell pp.54-62<br />
* ''Diploma course notes'' 1987 - Gem-A<br />
* ''Über die Eigenshaften von im Flussmittelverfahren hergestellten synthetischen roten und blauen Spinellen aus Russland'' - U. Henn/H. Bank, Gemmologie Jahrgang 41 / Heft 1 / April 1992, pp1-7<br />
* [http://nordskip.com/seriwat.html Black Spinel and other Black Gems of Thailand] Dr. Seriwat Saminpanya of Srinakharinwirot University, 2008</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Spinel&diff=7679Spinel2008-09-19T16:07:24Z<p>Doos: /* Refractometer */</p>
<hr />
<div>{{spinel}}<br />
[[Image:Badak1.JPG|left|framed|Badakshan spinel, Afghanistan]] <br clear="left" /><br />
{{images}}<br />
<br />
Spinel is a mineral group. For many centuries, most gem spinels were misidentified as [[sapphire]] or [[ruby]] because they have similar properties and occur in the same geological deposits. The historically significant 5.08 centimeter "Black Prince Ruby" in the center of the British Imperial Crown was only recently identified as a spinel. This stone is irregular in shape and has a somewhat squareish outline. Additionally, it was not faceted, merely polished. Spinels also occur in a vast array of colors. They are slightly softer than sapphires but still very durable.<br>The earliest known use of spinels was as ornaments found in Buddhist tombs in Afghanistan. Blue spinels have been found in England, dating back to the Roman occupation (51 BC to 400 AD).<br />
<br />
<!-- [[Image:DSC_7858_d.jpg|left|thumb|250px|Cobalt Blue Spinel, 3.96 ct <br /> Photo by Jeff Scovil<br />Courtesy of R.W. Wise Goldsmiths]] --><br />
<br clear="all" /> <br />
<br />
==Chemical composition==<br />
<br />
Common spinel belongs to the "spinel series", which belongs to the "spinel group".<br /><br />
The general formula for the spinel group is A<sup>2+</sup>B<sup>3+</sup><sub>2</sub>O<sub>4</sub>. The 3 series of the spinel group are defined by the B<sup>3+</sup> cation.<br /><br />
The spinel group is made up of 3 [[isomorphous replacement|isomorphous]] series.<br />
<br />
The isomorphous series:<br />
* Spinel series (aluminum)<br />
** Spinel - MgAl<sub>2</sub>O<sub>4</sub> (n = 1.719, sg ~ 3.60)<br />
** Hercynite - FeAl<sub>2</sub>O<sub>4</sub><br />
** Gahnite - ZnAl<sub>2</sub>O<sub>4</sub> (n = 1.805, sg = 4.62)<br />
** Galaxite - MnAl<sub>2</sub>O<sub>4</sub><br />
<br />
* Magnetite series (ferric iron)<br />
** Magnetite - FeFe<sub>2</sub>O<sub>4</sub><br />
** Magnesioferrite - MgFe<sub>2</sub>O<sub>4</sub><br />
** Ulvöspinel - FeFeTiO<sub>4</sub><br />
** Franklinite - ZnFe<sub>2</sub>O<sub>4</sub><br />
** Jacobsite - MnFe<sub>2</sub>O<sub>4</sub><br />
** Trevorite - NiFe<sub>2</sub>O<sub>4</sub><br />
<br />
* Chromite series (chrome)<br />
** Chromite - FeCr<sub>2</sub>O<sub>4</sub><br />
** Magnesiochromite - MgCr<sub>2</sub>O<sub>4</sub><br />
<br />
Most of the above series members are rare in nature with the exception of the members of the spinel series, magnetite and chromite. To gemologists common spinel and gahnite are of most interest.<br />
<br />
When gemologist refer to "spinel", we usually imply common spinel, that is the spinel that belongs to the spinel series of the spinel group.<br />
<br />
==Diagnostics==<br />
<br />
Spinel can be confused with many stones by appearance alone, yet optical properties usually rule out most of them.<br /><br />
As spinel belongs to an isomorhous series, the optical and physical properties may vary.<br />
<br />
===Color===<br />
<br />
Spinel: colorless, green, blue, red, black.<br /><br />
Gahnite: blue-green, yellow, brown.<br />
<br />
Varieties:<br />
*Pleonaste (also named ceylonite) - (Mg,Fe)Al<sub>2</sub>0<sub>4</sub> - dark green to blue-green; black<br />
*Gahnospinel - (Mg,Zn)Al<sub>2</sub>0<sub>4</sub> - pale to dark blue, black.<br />
<br />
Spinel is allochromatic and variety colors are produced by transition metals:<br />
* Cr<sup>3+</sup> - red, pink<br />
* Fe<sup>2+</sup> - blue, violet<br />
* Fe<sup>2+</sup> + Co<sup>2+</sup> - darkblue<br />
* Fe<sup>3+</sup> - green<br />
<br />
===Diaphaneity===<br />
<br />
Transparent to opaque.<br />
<br />
===Refractometer===<br />
<br />
Spinel is isotropic and the refractive index of common spinel is generally around 1.712 to 1.720. Red spinel can have a refractive index up to 1.74.<br /><br />
The only other isotropic gemstone that falls within this range is grossular garnet, but it will usually be higher and the color is also different.<br /><br />
Other members of the spinel series, such as gahnite will have higher refractive indices.<br />
<br />
All other gemstones can easily be seperated from spinel by their optic nature.<br />
<br />
Synthetic spinel has a usual refractive index of 1.727.<br /><br />
Gahnospinels have a refractive index between 1.725 and 1.753, while pleonaste (ceylonite) has an R.I. range of 1.77 to 1.80.<br /><br />
Black spinel-hercynite (pleonaste) stones from Thailand may have an RI upto 1.789 with an average SG of 3.86 (Seriwat, 2008)<br />
<br />
===Specific gravity===<br />
<br />
As with the refractive indices, the specif gravity of spinel can vary due to isomorphous replacement.<br /><br />
The values for most gem grade material lies between 3.58 and 3.61. Pleonaste has a S.G. between 3.63 and 3.90.<br /><br />
Gahnospinels may have a specific gravity up to 4.06.<br />
<br />
===Polariscope===<br />
<br />
Common spinel is isotrope and will remain dark under crossed polars.<br /><br />
Verneuil type synthetic spinel will always (maybe with the exception of red) show strong anomalous birefringence due to excess Al<sub>2</sub>O<sub>3</sub> (see [[spinel#synthetics|synthetics]]). This anomalous extinction (as it is currently named) can be seen as "tabby" extinction, resembling the color distribution of a cat's fur, or/and as an Andreas cross caused by pseudo-birefringence.<br />
<br />
Natural and flux-melt synthetic spinel may show weak anomalous extinction.<br />
<br />
===Spectra===<br />
<br />
[[Image:Spectrum natural blue spinel iron.jpg|thumb|left|240px]]<br />Spectrum of a natural blue spinel, colored by iron and minor traces of cobalt.<br clear="all" /><br />
<br />
==Phenomena==<br />
<br />
* Asterism (4 and 6-pointed stars)<br />
* Color change (rare)<br />
<br />
==Occurrence==<br />
<br />
Myanmar; Vietnam; Thailand; Sri Lanka; Pakistan; Afghanistan; Tadshikistan; Kenia; Tanzania; South-Africa; Brazil.<br />
<br />
==Synthetics==<br />
<br />
Spinel is synthesized by the [[flame fusion|Verneuil (flame-fusion) process]] and the flux-melt method, although the first process does not render a true synthetic in most cases.<br />
<br />
===Flame fusion===<br />
<br />
Flame fusion synthetic spinels are produced since 1908 (by accident while creating synthetic corundum), but were not commercially available until 1930.<br />
<br />
It was found that while trying to synthesize spinel through the Verneuil process, the resulting boules would easily fracture and no reasonably sized gemstones could be cut from them.<br /><br />
The ratio MgO to Al<sub>2</sub>O<sub>3</sub> is 1:1 for common spinel and by changing that ratio (adding Al<sub>2</sub>O<sub>3</sub>) the boules became stable. Because that alters the chemical formula of the synthetic, it is not a true synthetic (but accepted as such). Sometimes these are named "beta-corundum" due to the excess of alumnia.<br />
<br />
For the creation of red stones, this alteration was no option and, usually, no red synthetic spinel boules created by the flame-fusion process result in large stones (but are known to excist). The larger sized red synthetic spinels are mostly created with the flux-melt method. The few red synthetics that are created through the Verneuil process will show curved striae like their synthetic corundum cousins.<br />
<br />
As a result in the changing of the MgO:Al<sub>2</sub>O<sub>3</sub> ratio, the flame fusion synthetics have higher refractive indices (usually stable at 1.727) and a higher specific gravity (3.64).<br />
<br />
{| {{table}} width="70%" style="margin-left:0;"<br />
|-<br />
|colspan="5" align="center"|Properties of flame fusion synthetic spinel<br />
|-<br />
!color<br />
!coloring agent<br />
!RI<br />
!SG<br />
!other diagnostics<br />
|-<br />
|colorless||none||1.728-1.740||3.65-3.80||LWUV: green; SWUV: white/blue<br />
|-<br />
|red||Cr<sup>3+</sup>||1.722-1.725*||3.58-3.60||curved striae; spectrum; fluorescence<br />
|- <br />
|pink||Cu||1.727-1.740||3.65-3.80<br />
|-<br />
|yellow||Mn||do.||do.||LW/SWUV: green<br />
|-<br />
|emerald green||Mn + Co<sup>3+</sup>||do.||do.<br />
|-<br />
|tourmaline green||Cr<sup>3+</sup>||do.||do.||spectrum<br />
|-<br />
|beryl green||Cr<sup>3+</sup> + Mn||do.||do.||spectrum<br />
|-<br />
|zircon blue||Co<sup>3+</sup> + Cr<sup>3+</sup> + Ti||do.||do.||spectrum<br />
|-<br />
|sapphire blue||Co<sup>3+</sup>||do.||do.||spectrum; fluorescence; CCF<br />
|-<br />
|amethyst violet||Co<sup>3+</sup> + Mn||do.||do.<br />
|-<br />
|alexandrite color change||Cr<sup>3+</sup> + V||do.||do.||spectrum<br />
|-<br />
|lapis lazuli||Co<sup>3+</sup>||1.725||3.52||spectrum; CCF<br />
|-<br />
|colspan="5"|<br />
Strong "tabby" extinction can be seen in all<br /><br />
<nowiki>*</nowiki> Henn, 1995 metions a low value of 1.720.<br />
|}<br />
<br />
===Flux-melt===<br />
<br />
First commercial production of flux-melt synthetic spinel started around 1980, although successful experiments date back to 1848 (by the French chemist Ebelmen). As of 1989 larger volumes of this synthetic appeared on the market, produced in Novosibirsk, Russia.<br />
<br />
Apart from red, blue synthetics have also been produced by the flux-melt process.<br /><br />
These are true synthetics.<br />
<br />
{| {{table}} width="70%" style="margin-left:0;"<br />
|-<br />
|colspan="5" align="center"|Properties of flux-melt synthetic spinel<br />
|-<br />
!color<br />
!coloring agent<br />
!RI<br />
!SG<br />
!other diagnostics<br />
|-<br />
|red||Cr<sup>3+</sup>||1.716-1.719||3.58-3.62||LW/SWUV: distinct red-orange/red<br />flux residu inclusions<br />
|-<br />
|blue||Co<sup>2+</sup> + Fe<sup>2+</sup>||1.719||3.58||spectrum; SWUV: inert; LWUV: weak read<br />flux residu inclusions<br />
|}<br />
Tabby extinction is also seen in these synthetics. Careful observation of inclusions is the main means of separation for red stones. Blue flux-melt synthetics can also be distinguished by the spectrum.<br />
<br />
===Czochralski pulling method===<br />
A recent development (2007) is the procuction of red to pink synthetic spinels by the Czochralski pulling method.<br />
<br />
==Inclusion images==<br />
<br />
[[image:SP713Fzm1brE2U.jpg|thumb|left|240px|Apatite inclusions and their star-like outgrowths along the 60 degree hexagonal plane - looking much like stellate dislocation systems, in a cobalt blue spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
[[image:SP820AO2-16.jpg|thumb|left|240px|Octahedral spinel inclusions with "Saturn-ring" stress fractures in a Burma red spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
[[image:SP820A02colW.jpg|thumb|left|240px|Octahedral inclusions in a Burma red spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
<br clear="all" /><br />
{{inclusions}}<br />
<br />
==Sources==<br />
* ''Introduction to Optical Mineralogy'' 2004 - William D. Nesse ISBN 0195149106<br />
* ''Gemmology 3rd edition'' (2005) - Peter G. Read ISBN 0750664495<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''Edelsteinkuntliches Praktikum'' - Ulrich Henn, Gemmologie Jahrgang 44 / Heft 4 / Dezember 1995, Spinell pp.54-62<br />
* ''Diploma course notes'' 1987 - Gem-A<br />
* ''Über die Eigenshaften von im Flussmittelverfahren hergestellten synthetischen roten und blauen Spinellen aus Russland'' - U. Henn/H. Bank, Gemmologie Jahrgang 41 / Heft 1 / April 1992, pp1-7</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Spinel&diff=7678Spinel2008-09-19T16:05:10Z<p>Doos: /* Refractometer */</p>
<hr />
<div>{{spinel}}<br />
[[Image:Badak1.JPG|left|framed|Badakshan spinel, Afghanistan]] <br clear="left" /><br />
{{images}}<br />
<br />
Spinel is a mineral group. For many centuries, most gem spinels were misidentified as [[sapphire]] or [[ruby]] because they have similar properties and occur in the same geological deposits. The historically significant 5.08 centimeter "Black Prince Ruby" in the center of the British Imperial Crown was only recently identified as a spinel. This stone is irregular in shape and has a somewhat squareish outline. Additionally, it was not faceted, merely polished. Spinels also occur in a vast array of colors. They are slightly softer than sapphires but still very durable.<br>The earliest known use of spinels was as ornaments found in Buddhist tombs in Afghanistan. Blue spinels have been found in England, dating back to the Roman occupation (51 BC to 400 AD).<br />
<br />
<!-- [[Image:DSC_7858_d.jpg|left|thumb|250px|Cobalt Blue Spinel, 3.96 ct <br /> Photo by Jeff Scovil<br />Courtesy of R.W. Wise Goldsmiths]] --><br />
<br clear="all" /> <br />
<br />
==Chemical composition==<br />
<br />
Common spinel belongs to the "spinel series", which belongs to the "spinel group".<br /><br />
The general formula for the spinel group is A<sup>2+</sup>B<sup>3+</sup><sub>2</sub>O<sub>4</sub>. The 3 series of the spinel group are defined by the B<sup>3+</sup> cation.<br /><br />
The spinel group is made up of 3 [[isomorphous replacement|isomorphous]] series.<br />
<br />
The isomorphous series:<br />
* Spinel series (aluminum)<br />
** Spinel - MgAl<sub>2</sub>O<sub>4</sub> (n = 1.719, sg ~ 3.60)<br />
** Hercynite - FeAl<sub>2</sub>O<sub>4</sub><br />
** Gahnite - ZnAl<sub>2</sub>O<sub>4</sub> (n = 1.805, sg = 4.62)<br />
** Galaxite - MnAl<sub>2</sub>O<sub>4</sub><br />
<br />
* Magnetite series (ferric iron)<br />
** Magnetite - FeFe<sub>2</sub>O<sub>4</sub><br />
** Magnesioferrite - MgFe<sub>2</sub>O<sub>4</sub><br />
** Ulvöspinel - FeFeTiO<sub>4</sub><br />
** Franklinite - ZnFe<sub>2</sub>O<sub>4</sub><br />
** Jacobsite - MnFe<sub>2</sub>O<sub>4</sub><br />
** Trevorite - NiFe<sub>2</sub>O<sub>4</sub><br />
<br />
* Chromite series (chrome)<br />
** Chromite - FeCr<sub>2</sub>O<sub>4</sub><br />
** Magnesiochromite - MgCr<sub>2</sub>O<sub>4</sub><br />
<br />
Most of the above series members are rare in nature with the exception of the members of the spinel series, magnetite and chromite. To gemologists common spinel and gahnite are of most interest.<br />
<br />
When gemologist refer to "spinel", we usually imply common spinel, that is the spinel that belongs to the spinel series of the spinel group.<br />
<br />
==Diagnostics==<br />
<br />
Spinel can be confused with many stones by appearance alone, yet optical properties usually rule out most of them.<br /><br />
As spinel belongs to an isomorhous series, the optical and physical properties may vary.<br />
<br />
===Color===<br />
<br />
Spinel: colorless, green, blue, red, black.<br /><br />
Gahnite: blue-green, yellow, brown.<br />
<br />
Varieties:<br />
*Pleonaste (also named ceylonite) - (Mg,Fe)Al<sub>2</sub>0<sub>4</sub> - dark green to blue-green; black<br />
*Gahnospinel - (Mg,Zn)Al<sub>2</sub>0<sub>4</sub> - pale to dark blue, black.<br />
<br />
Spinel is allochromatic and variety colors are produced by transition metals:<br />
* Cr<sup>3+</sup> - red, pink<br />
* Fe<sup>2+</sup> - blue, violet<br />
* Fe<sup>2+</sup> + Co<sup>2+</sup> - darkblue<br />
* Fe<sup>3+</sup> - green<br />
<br />
===Diaphaneity===<br />
<br />
Transparent to opaque.<br />
<br />
===Refractometer===<br />
<br />
Spinel is isotropic and the refractive index of common spinel is generally around 1.712 to 1.720. Red spinel can have a refractive index up to 1.74.<br /><br />
The only other isotropic gemstone that falls within this range is grossular garnet, but it will usually be higher and the color is also different.<br /><br />
Other members of the spinel series, such as gahnite will have higher refractive indices.<br />
<br />
All other gemstones can easily be seperated from spinel by their optic nature.<br />
<br />
Synthetic spinel has a usual refractive index of 1.727.<br /><br />
Gahnospinels have a refractive index between 1.725 and 1.753, while pleonaste (ceylonite) has an R.I. range of 1.77 to 1.80.<br /><br />
Black spinel-hercynite (pleonaste) stones from Thailand may have an RI upto 1.789 with an average SG of 3.86.<br />
<br />
===Specific gravity===<br />
<br />
As with the refractive indices, the specif gravity of spinel can vary due to isomorphous replacement.<br /><br />
The values for most gem grade material lies between 3.58 and 3.61. Pleonaste has a S.G. between 3.63 and 3.90.<br /><br />
Gahnospinels may have a specific gravity up to 4.06.<br />
<br />
===Polariscope===<br />
<br />
Common spinel is isotrope and will remain dark under crossed polars.<br /><br />
Verneuil type synthetic spinel will always (maybe with the exception of red) show strong anomalous birefringence due to excess Al<sub>2</sub>O<sub>3</sub> (see [[spinel#synthetics|synthetics]]). This anomalous extinction (as it is currently named) can be seen as "tabby" extinction, resembling the color distribution of a cat's fur, or/and as an Andreas cross caused by pseudo-birefringence.<br />
<br />
Natural and flux-melt synthetic spinel may show weak anomalous extinction.<br />
<br />
===Spectra===<br />
<br />
[[Image:Spectrum natural blue spinel iron.jpg|thumb|left|240px]]<br />Spectrum of a natural blue spinel, colored by iron and minor traces of cobalt.<br clear="all" /><br />
<br />
==Phenomena==<br />
<br />
* Asterism (4 and 6-pointed stars)<br />
* Color change (rare)<br />
<br />
==Occurrence==<br />
<br />
Myanmar; Vietnam; Thailand; Sri Lanka; Pakistan; Afghanistan; Tadshikistan; Kenia; Tanzania; South-Africa; Brazil.<br />
<br />
==Synthetics==<br />
<br />
Spinel is synthesized by the [[flame fusion|Verneuil (flame-fusion) process]] and the flux-melt method, although the first process does not render a true synthetic in most cases.<br />
<br />
===Flame fusion===<br />
<br />
Flame fusion synthetic spinels are produced since 1908 (by accident while creating synthetic corundum), but were not commercially available until 1930.<br />
<br />
It was found that while trying to synthesize spinel through the Verneuil process, the resulting boules would easily fracture and no reasonably sized gemstones could be cut from them.<br /><br />
The ratio MgO to Al<sub>2</sub>O<sub>3</sub> is 1:1 for common spinel and by changing that ratio (adding Al<sub>2</sub>O<sub>3</sub>) the boules became stable. Because that alters the chemical formula of the synthetic, it is not a true synthetic (but accepted as such). Sometimes these are named "beta-corundum" due to the excess of alumnia.<br />
<br />
For the creation of red stones, this alteration was no option and, usually, no red synthetic spinel boules created by the flame-fusion process result in large stones (but are known to excist). The larger sized red synthetic spinels are mostly created with the flux-melt method. The few red synthetics that are created through the Verneuil process will show curved striae like their synthetic corundum cousins.<br />
<br />
As a result in the changing of the MgO:Al<sub>2</sub>O<sub>3</sub> ratio, the flame fusion synthetics have higher refractive indices (usually stable at 1.727) and a higher specific gravity (3.64).<br />
<br />
{| {{table}} width="70%" style="margin-left:0;"<br />
|-<br />
|colspan="5" align="center"|Properties of flame fusion synthetic spinel<br />
|-<br />
!color<br />
!coloring agent<br />
!RI<br />
!SG<br />
!other diagnostics<br />
|-<br />
|colorless||none||1.728-1.740||3.65-3.80||LWUV: green; SWUV: white/blue<br />
|-<br />
|red||Cr<sup>3+</sup>||1.722-1.725*||3.58-3.60||curved striae; spectrum; fluorescence<br />
|- <br />
|pink||Cu||1.727-1.740||3.65-3.80<br />
|-<br />
|yellow||Mn||do.||do.||LW/SWUV: green<br />
|-<br />
|emerald green||Mn + Co<sup>3+</sup>||do.||do.<br />
|-<br />
|tourmaline green||Cr<sup>3+</sup>||do.||do.||spectrum<br />
|-<br />
|beryl green||Cr<sup>3+</sup> + Mn||do.||do.||spectrum<br />
|-<br />
|zircon blue||Co<sup>3+</sup> + Cr<sup>3+</sup> + Ti||do.||do.||spectrum<br />
|-<br />
|sapphire blue||Co<sup>3+</sup>||do.||do.||spectrum; fluorescence; CCF<br />
|-<br />
|amethyst violet||Co<sup>3+</sup> + Mn||do.||do.<br />
|-<br />
|alexandrite color change||Cr<sup>3+</sup> + V||do.||do.||spectrum<br />
|-<br />
|lapis lazuli||Co<sup>3+</sup>||1.725||3.52||spectrum; CCF<br />
|-<br />
|colspan="5"|<br />
Strong "tabby" extinction can be seen in all<br /><br />
<nowiki>*</nowiki> Henn, 1995 metions a low value of 1.720.<br />
|}<br />
<br />
===Flux-melt===<br />
<br />
First commercial production of flux-melt synthetic spinel started around 1980, although successful experiments date back to 1848 (by the French chemist Ebelmen). As of 1989 larger volumes of this synthetic appeared on the market, produced in Novosibirsk, Russia.<br />
<br />
Apart from red, blue synthetics have also been produced by the flux-melt process.<br /><br />
These are true synthetics.<br />
<br />
{| {{table}} width="70%" style="margin-left:0;"<br />
|-<br />
|colspan="5" align="center"|Properties of flux-melt synthetic spinel<br />
|-<br />
!color<br />
!coloring agent<br />
!RI<br />
!SG<br />
!other diagnostics<br />
|-<br />
|red||Cr<sup>3+</sup>||1.716-1.719||3.58-3.62||LW/SWUV: distinct red-orange/red<br />flux residu inclusions<br />
|-<br />
|blue||Co<sup>2+</sup> + Fe<sup>2+</sup>||1.719||3.58||spectrum; SWUV: inert; LWUV: weak read<br />flux residu inclusions<br />
|}<br />
Tabby extinction is also seen in these synthetics. Careful observation of inclusions is the main means of separation for red stones. Blue flux-melt synthetics can also be distinguished by the spectrum.<br />
<br />
===Czochralski pulling method===<br />
A recent development (2007) is the procuction of red to pink synthetic spinels by the Czochralski pulling method.<br />
<br />
==Inclusion images==<br />
<br />
[[image:SP713Fzm1brE2U.jpg|thumb|left|240px|Apatite inclusions and their star-like outgrowths along the 60 degree hexagonal plane - looking much like stellate dislocation systems, in a cobalt blue spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
[[image:SP820AO2-16.jpg|thumb|left|240px|Octahedral spinel inclusions with "Saturn-ring" stress fractures in a Burma red spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
[[image:SP820A02colW.jpg|thumb|left|240px|Octahedral inclusions in a Burma red spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
<br clear="all" /><br />
{{inclusions}}<br />
<br />
==Sources==<br />
* ''Introduction to Optical Mineralogy'' 2004 - William D. Nesse ISBN 0195149106<br />
* ''Gemmology 3rd edition'' (2005) - Peter G. Read ISBN 0750664495<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''Edelsteinkuntliches Praktikum'' - Ulrich Henn, Gemmologie Jahrgang 44 / Heft 4 / Dezember 1995, Spinell pp.54-62<br />
* ''Diploma course notes'' 1987 - Gem-A<br />
* ''Über die Eigenshaften von im Flussmittelverfahren hergestellten synthetischen roten und blauen Spinellen aus Russland'' - U. Henn/H. Bank, Gemmologie Jahrgang 41 / Heft 1 / April 1992, pp1-7</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Spinel&diff=7677Spinel2008-09-19T16:00:04Z<p>Doos: /* Color */</p>
<hr />
<div>{{spinel}}<br />
[[Image:Badak1.JPG|left|framed|Badakshan spinel, Afghanistan]] <br clear="left" /><br />
{{images}}<br />
<br />
Spinel is a mineral group. For many centuries, most gem spinels were misidentified as [[sapphire]] or [[ruby]] because they have similar properties and occur in the same geological deposits. The historically significant 5.08 centimeter "Black Prince Ruby" in the center of the British Imperial Crown was only recently identified as a spinel. This stone is irregular in shape and has a somewhat squareish outline. Additionally, it was not faceted, merely polished. Spinels also occur in a vast array of colors. They are slightly softer than sapphires but still very durable.<br>The earliest known use of spinels was as ornaments found in Buddhist tombs in Afghanistan. Blue spinels have been found in England, dating back to the Roman occupation (51 BC to 400 AD).<br />
<br />
<!-- [[Image:DSC_7858_d.jpg|left|thumb|250px|Cobalt Blue Spinel, 3.96 ct <br /> Photo by Jeff Scovil<br />Courtesy of R.W. Wise Goldsmiths]] --><br />
<br clear="all" /> <br />
<br />
==Chemical composition==<br />
<br />
Common spinel belongs to the "spinel series", which belongs to the "spinel group".<br /><br />
The general formula for the spinel group is A<sup>2+</sup>B<sup>3+</sup><sub>2</sub>O<sub>4</sub>. The 3 series of the spinel group are defined by the B<sup>3+</sup> cation.<br /><br />
The spinel group is made up of 3 [[isomorphous replacement|isomorphous]] series.<br />
<br />
The isomorphous series:<br />
* Spinel series (aluminum)<br />
** Spinel - MgAl<sub>2</sub>O<sub>4</sub> (n = 1.719, sg ~ 3.60)<br />
** Hercynite - FeAl<sub>2</sub>O<sub>4</sub><br />
** Gahnite - ZnAl<sub>2</sub>O<sub>4</sub> (n = 1.805, sg = 4.62)<br />
** Galaxite - MnAl<sub>2</sub>O<sub>4</sub><br />
<br />
* Magnetite series (ferric iron)<br />
** Magnetite - FeFe<sub>2</sub>O<sub>4</sub><br />
** Magnesioferrite - MgFe<sub>2</sub>O<sub>4</sub><br />
** Ulvöspinel - FeFeTiO<sub>4</sub><br />
** Franklinite - ZnFe<sub>2</sub>O<sub>4</sub><br />
** Jacobsite - MnFe<sub>2</sub>O<sub>4</sub><br />
** Trevorite - NiFe<sub>2</sub>O<sub>4</sub><br />
<br />
* Chromite series (chrome)<br />
** Chromite - FeCr<sub>2</sub>O<sub>4</sub><br />
** Magnesiochromite - MgCr<sub>2</sub>O<sub>4</sub><br />
<br />
Most of the above series members are rare in nature with the exception of the members of the spinel series, magnetite and chromite. To gemologists common spinel and gahnite are of most interest.<br />
<br />
When gemologist refer to "spinel", we usually imply common spinel, that is the spinel that belongs to the spinel series of the spinel group.<br />
<br />
==Diagnostics==<br />
<br />
Spinel can be confused with many stones by appearance alone, yet optical properties usually rule out most of them.<br /><br />
As spinel belongs to an isomorhous series, the optical and physical properties may vary.<br />
<br />
===Color===<br />
<br />
Spinel: colorless, green, blue, red, black.<br /><br />
Gahnite: blue-green, yellow, brown.<br />
<br />
Varieties:<br />
*Pleonaste (also named ceylonite) - (Mg,Fe)Al<sub>2</sub>0<sub>4</sub> - dark green to blue-green; black<br />
*Gahnospinel - (Mg,Zn)Al<sub>2</sub>0<sub>4</sub> - pale to dark blue, black.<br />
<br />
Spinel is allochromatic and variety colors are produced by transition metals:<br />
* Cr<sup>3+</sup> - red, pink<br />
* Fe<sup>2+</sup> - blue, violet<br />
* Fe<sup>2+</sup> + Co<sup>2+</sup> - darkblue<br />
* Fe<sup>3+</sup> - green<br />
<br />
===Diaphaneity===<br />
<br />
Transparent to opaque.<br />
<br />
===Refractometer===<br />
<br />
Spinel is isotropic and the refractive index of common spinel is generally around 1.712 to 1.720. Red spinel can have a refractive index up to 1.74.<br /><br />
The only other isotropic gemstone that falls within this range is grossular garnet, but it will usually be higher and the color is also different.<br /><br />
Other members of the spinel series, such as gahnite will have higher refractive indices.<br />
<br />
All other gemstones can easily be seperated from spinel by their optic nature.<br />
<br />
Synthetic spinel has a usual refractive index of 1.727.<br /><br />
Gahnospinels have a refractive index between 1.725 and 1.753, while pleonaste (ceylonite) has an R.I. range of 1.77 to 1.80.<br /><br />
Black spinel-hercynite stones from Thailand may have an RI upto 1.789 with an average SG of 3.86.<br />
<br />
===Specific gravity===<br />
<br />
As with the refractive indices, the specif gravity of spinel can vary due to isomorphous replacement.<br /><br />
The values for most gem grade material lies between 3.58 and 3.61. Pleonaste has a S.G. between 3.63 and 3.90.<br /><br />
Gahnospinels may have a specific gravity up to 4.06.<br />
<br />
===Polariscope===<br />
<br />
Common spinel is isotrope and will remain dark under crossed polars.<br /><br />
Verneuil type synthetic spinel will always (maybe with the exception of red) show strong anomalous birefringence due to excess Al<sub>2</sub>O<sub>3</sub> (see [[spinel#synthetics|synthetics]]). This anomalous extinction (as it is currently named) can be seen as "tabby" extinction, resembling the color distribution of a cat's fur, or/and as an Andreas cross caused by pseudo-birefringence.<br />
<br />
Natural and flux-melt synthetic spinel may show weak anomalous extinction.<br />
<br />
===Spectra===<br />
<br />
[[Image:Spectrum natural blue spinel iron.jpg|thumb|left|240px]]<br />Spectrum of a natural blue spinel, colored by iron and minor traces of cobalt.<br clear="all" /><br />
<br />
==Phenomena==<br />
<br />
* Asterism (4 and 6-pointed stars)<br />
* Color change (rare)<br />
<br />
==Occurrence==<br />
<br />
Myanmar; Vietnam; Thailand; Sri Lanka; Pakistan; Afghanistan; Tadshikistan; Kenia; Tanzania; South-Africa; Brazil.<br />
<br />
==Synthetics==<br />
<br />
Spinel is synthesized by the [[flame fusion|Verneuil (flame-fusion) process]] and the flux-melt method, although the first process does not render a true synthetic in most cases.<br />
<br />
===Flame fusion===<br />
<br />
Flame fusion synthetic spinels are produced since 1908 (by accident while creating synthetic corundum), but were not commercially available until 1930.<br />
<br />
It was found that while trying to synthesize spinel through the Verneuil process, the resulting boules would easily fracture and no reasonably sized gemstones could be cut from them.<br /><br />
The ratio MgO to Al<sub>2</sub>O<sub>3</sub> is 1:1 for common spinel and by changing that ratio (adding Al<sub>2</sub>O<sub>3</sub>) the boules became stable. Because that alters the chemical formula of the synthetic, it is not a true synthetic (but accepted as such). Sometimes these are named "beta-corundum" due to the excess of alumnia.<br />
<br />
For the creation of red stones, this alteration was no option and, usually, no red synthetic spinel boules created by the flame-fusion process result in large stones (but are known to excist). The larger sized red synthetic spinels are mostly created with the flux-melt method. The few red synthetics that are created through the Verneuil process will show curved striae like their synthetic corundum cousins.<br />
<br />
As a result in the changing of the MgO:Al<sub>2</sub>O<sub>3</sub> ratio, the flame fusion synthetics have higher refractive indices (usually stable at 1.727) and a higher specific gravity (3.64).<br />
<br />
{| {{table}} width="70%" style="margin-left:0;"<br />
|-<br />
|colspan="5" align="center"|Properties of flame fusion synthetic spinel<br />
|-<br />
!color<br />
!coloring agent<br />
!RI<br />
!SG<br />
!other diagnostics<br />
|-<br />
|colorless||none||1.728-1.740||3.65-3.80||LWUV: green; SWUV: white/blue<br />
|-<br />
|red||Cr<sup>3+</sup>||1.722-1.725*||3.58-3.60||curved striae; spectrum; fluorescence<br />
|- <br />
|pink||Cu||1.727-1.740||3.65-3.80<br />
|-<br />
|yellow||Mn||do.||do.||LW/SWUV: green<br />
|-<br />
|emerald green||Mn + Co<sup>3+</sup>||do.||do.<br />
|-<br />
|tourmaline green||Cr<sup>3+</sup>||do.||do.||spectrum<br />
|-<br />
|beryl green||Cr<sup>3+</sup> + Mn||do.||do.||spectrum<br />
|-<br />
|zircon blue||Co<sup>3+</sup> + Cr<sup>3+</sup> + Ti||do.||do.||spectrum<br />
|-<br />
|sapphire blue||Co<sup>3+</sup>||do.||do.||spectrum; fluorescence; CCF<br />
|-<br />
|amethyst violet||Co<sup>3+</sup> + Mn||do.||do.<br />
|-<br />
|alexandrite color change||Cr<sup>3+</sup> + V||do.||do.||spectrum<br />
|-<br />
|lapis lazuli||Co<sup>3+</sup>||1.725||3.52||spectrum; CCF<br />
|-<br />
|colspan="5"|<br />
Strong "tabby" extinction can be seen in all<br /><br />
<nowiki>*</nowiki> Henn, 1995 metions a low value of 1.720.<br />
|}<br />
<br />
===Flux-melt===<br />
<br />
First commercial production of flux-melt synthetic spinel started around 1980, although successful experiments date back to 1848 (by the French chemist Ebelmen). As of 1989 larger volumes of this synthetic appeared on the market, produced in Novosibirsk, Russia.<br />
<br />
Apart from red, blue synthetics have also been produced by the flux-melt process.<br /><br />
These are true synthetics.<br />
<br />
{| {{table}} width="70%" style="margin-left:0;"<br />
|-<br />
|colspan="5" align="center"|Properties of flux-melt synthetic spinel<br />
|-<br />
!color<br />
!coloring agent<br />
!RI<br />
!SG<br />
!other diagnostics<br />
|-<br />
|red||Cr<sup>3+</sup>||1.716-1.719||3.58-3.62||LW/SWUV: distinct red-orange/red<br />flux residu inclusions<br />
|-<br />
|blue||Co<sup>2+</sup> + Fe<sup>2+</sup>||1.719||3.58||spectrum; SWUV: inert; LWUV: weak read<br />flux residu inclusions<br />
|}<br />
Tabby extinction is also seen in these synthetics. Careful observation of inclusions is the main means of separation for red stones. Blue flux-melt synthetics can also be distinguished by the spectrum.<br />
<br />
===Czochralski pulling method===<br />
A recent development (2007) is the procuction of red to pink synthetic spinels by the Czochralski pulling method.<br />
<br />
==Inclusion images==<br />
<br />
[[image:SP713Fzm1brE2U.jpg|thumb|left|240px|Apatite inclusions and their star-like outgrowths along the 60 degree hexagonal plane - looking much like stellate dislocation systems, in a cobalt blue spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
[[image:SP820AO2-16.jpg|thumb|left|240px|Octahedral spinel inclusions with "Saturn-ring" stress fractures in a Burma red spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
[[image:SP820A02colW.jpg|thumb|left|240px|Octahedral inclusions in a Burma red spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
<br clear="all" /><br />
{{inclusions}}<br />
<br />
==Sources==<br />
* ''Introduction to Optical Mineralogy'' 2004 - William D. Nesse ISBN 0195149106<br />
* ''Gemmology 3rd edition'' (2005) - Peter G. Read ISBN 0750664495<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''Edelsteinkuntliches Praktikum'' - Ulrich Henn, Gemmologie Jahrgang 44 / Heft 4 / Dezember 1995, Spinell pp.54-62<br />
* ''Diploma course notes'' 1987 - Gem-A<br />
* ''Über die Eigenshaften von im Flussmittelverfahren hergestellten synthetischen roten und blauen Spinellen aus Russland'' - U. Henn/H. Bank, Gemmologie Jahrgang 41 / Heft 1 / April 1992, pp1-7</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Spinel&diff=7676Spinel2008-09-19T15:59:05Z<p>Doos: /* Refractometer */</p>
<hr />
<div>{{spinel}}<br />
[[Image:Badak1.JPG|left|framed|Badakshan spinel, Afghanistan]] <br clear="left" /><br />
{{images}}<br />
<br />
Spinel is a mineral group. For many centuries, most gem spinels were misidentified as [[sapphire]] or [[ruby]] because they have similar properties and occur in the same geological deposits. The historically significant 5.08 centimeter "Black Prince Ruby" in the center of the British Imperial Crown was only recently identified as a spinel. This stone is irregular in shape and has a somewhat squareish outline. Additionally, it was not faceted, merely polished. Spinels also occur in a vast array of colors. They are slightly softer than sapphires but still very durable.<br>The earliest known use of spinels was as ornaments found in Buddhist tombs in Afghanistan. Blue spinels have been found in England, dating back to the Roman occupation (51 BC to 400 AD).<br />
<br />
<!-- [[Image:DSC_7858_d.jpg|left|thumb|250px|Cobalt Blue Spinel, 3.96 ct <br /> Photo by Jeff Scovil<br />Courtesy of R.W. Wise Goldsmiths]] --><br />
<br clear="all" /> <br />
<br />
==Chemical composition==<br />
<br />
Common spinel belongs to the "spinel series", which belongs to the "spinel group".<br /><br />
The general formula for the spinel group is A<sup>2+</sup>B<sup>3+</sup><sub>2</sub>O<sub>4</sub>. The 3 series of the spinel group are defined by the B<sup>3+</sup> cation.<br /><br />
The spinel group is made up of 3 [[isomorphous replacement|isomorphous]] series.<br />
<br />
The isomorphous series:<br />
* Spinel series (aluminum)<br />
** Spinel - MgAl<sub>2</sub>O<sub>4</sub> (n = 1.719, sg ~ 3.60)<br />
** Hercynite - FeAl<sub>2</sub>O<sub>4</sub><br />
** Gahnite - ZnAl<sub>2</sub>O<sub>4</sub> (n = 1.805, sg = 4.62)<br />
** Galaxite - MnAl<sub>2</sub>O<sub>4</sub><br />
<br />
* Magnetite series (ferric iron)<br />
** Magnetite - FeFe<sub>2</sub>O<sub>4</sub><br />
** Magnesioferrite - MgFe<sub>2</sub>O<sub>4</sub><br />
** Ulvöspinel - FeFeTiO<sub>4</sub><br />
** Franklinite - ZnFe<sub>2</sub>O<sub>4</sub><br />
** Jacobsite - MnFe<sub>2</sub>O<sub>4</sub><br />
** Trevorite - NiFe<sub>2</sub>O<sub>4</sub><br />
<br />
* Chromite series (chrome)<br />
** Chromite - FeCr<sub>2</sub>O<sub>4</sub><br />
** Magnesiochromite - MgCr<sub>2</sub>O<sub>4</sub><br />
<br />
Most of the above series members are rare in nature with the exception of the members of the spinel series, magnetite and chromite. To gemologists common spinel and gahnite are of most interest.<br />
<br />
When gemologist refer to "spinel", we usually imply common spinel, that is the spinel that belongs to the spinel series of the spinel group.<br />
<br />
==Diagnostics==<br />
<br />
Spinel can be confused with many stones by appearance alone, yet optical properties usually rule out most of them.<br /><br />
As spinel belongs to an isomorhous series, the optical and physical properties may vary.<br />
<br />
===Color===<br />
<br />
Spinel: colorless, green, blue, red, black.<br /><br />
Gahnite: blue-green, yellow, brown.<br />
<br />
Varieties:<br />
*Pleonaste (also named ceylonite) - (Mg,Fe)Al<sub>2</sub>0<sub>4</sub> - dark green to blue-green; black<br />
*Gahnospinel - (Mg,Zn)Al<sub>2</sub>0<sub>4</sub> - pale to dark blue<br />
<br />
Spinel is allochromatic and variety colors are produced by transition metals:<br />
* Cr<sup>3+</sup> - red, pink<br />
* Fe<sup>2+</sup> - blue, violet<br />
* Fe<sup>2+</sup> + Co<sup>2+</sup> - darkblue<br />
* Fe<sup>3+</sup> - green<br />
<br />
===Diaphaneity===<br />
<br />
Transparent to opaque.<br />
<br />
===Refractometer===<br />
<br />
Spinel is isotropic and the refractive index of common spinel is generally around 1.712 to 1.720. Red spinel can have a refractive index up to 1.74.<br /><br />
The only other isotropic gemstone that falls within this range is grossular garnet, but it will usually be higher and the color is also different.<br /><br />
Other members of the spinel series, such as gahnite will have higher refractive indices.<br />
<br />
All other gemstones can easily be seperated from spinel by their optic nature.<br />
<br />
Synthetic spinel has a usual refractive index of 1.727.<br /><br />
Gahnospinels have a refractive index between 1.725 and 1.753, while pleonaste (ceylonite) has an R.I. range of 1.77 to 1.80.<br /><br />
Black spinel-hercynite stones from Thailand may have an RI upto 1.789 with an average SG of 3.86.<br />
<br />
===Specific gravity===<br />
<br />
As with the refractive indices, the specif gravity of spinel can vary due to isomorphous replacement.<br /><br />
The values for most gem grade material lies between 3.58 and 3.61. Pleonaste has a S.G. between 3.63 and 3.90.<br /><br />
Gahnospinels may have a specific gravity up to 4.06.<br />
<br />
===Polariscope===<br />
<br />
Common spinel is isotrope and will remain dark under crossed polars.<br /><br />
Verneuil type synthetic spinel will always (maybe with the exception of red) show strong anomalous birefringence due to excess Al<sub>2</sub>O<sub>3</sub> (see [[spinel#synthetics|synthetics]]). This anomalous extinction (as it is currently named) can be seen as "tabby" extinction, resembling the color distribution of a cat's fur, or/and as an Andreas cross caused by pseudo-birefringence.<br />
<br />
Natural and flux-melt synthetic spinel may show weak anomalous extinction.<br />
<br />
===Spectra===<br />
<br />
[[Image:Spectrum natural blue spinel iron.jpg|thumb|left|240px]]<br />Spectrum of a natural blue spinel, colored by iron and minor traces of cobalt.<br clear="all" /><br />
<br />
==Phenomena==<br />
<br />
* Asterism (4 and 6-pointed stars)<br />
* Color change (rare)<br />
<br />
==Occurrence==<br />
<br />
Myanmar; Vietnam; Thailand; Sri Lanka; Pakistan; Afghanistan; Tadshikistan; Kenia; Tanzania; South-Africa; Brazil.<br />
<br />
==Synthetics==<br />
<br />
Spinel is synthesized by the [[flame fusion|Verneuil (flame-fusion) process]] and the flux-melt method, although the first process does not render a true synthetic in most cases.<br />
<br />
===Flame fusion===<br />
<br />
Flame fusion synthetic spinels are produced since 1908 (by accident while creating synthetic corundum), but were not commercially available until 1930.<br />
<br />
It was found that while trying to synthesize spinel through the Verneuil process, the resulting boules would easily fracture and no reasonably sized gemstones could be cut from them.<br /><br />
The ratio MgO to Al<sub>2</sub>O<sub>3</sub> is 1:1 for common spinel and by changing that ratio (adding Al<sub>2</sub>O<sub>3</sub>) the boules became stable. Because that alters the chemical formula of the synthetic, it is not a true synthetic (but accepted as such). Sometimes these are named "beta-corundum" due to the excess of alumnia.<br />
<br />
For the creation of red stones, this alteration was no option and, usually, no red synthetic spinel boules created by the flame-fusion process result in large stones (but are known to excist). The larger sized red synthetic spinels are mostly created with the flux-melt method. The few red synthetics that are created through the Verneuil process will show curved striae like their synthetic corundum cousins.<br />
<br />
As a result in the changing of the MgO:Al<sub>2</sub>O<sub>3</sub> ratio, the flame fusion synthetics have higher refractive indices (usually stable at 1.727) and a higher specific gravity (3.64).<br />
<br />
{| {{table}} width="70%" style="margin-left:0;"<br />
|-<br />
|colspan="5" align="center"|Properties of flame fusion synthetic spinel<br />
|-<br />
!color<br />
!coloring agent<br />
!RI<br />
!SG<br />
!other diagnostics<br />
|-<br />
|colorless||none||1.728-1.740||3.65-3.80||LWUV: green; SWUV: white/blue<br />
|-<br />
|red||Cr<sup>3+</sup>||1.722-1.725*||3.58-3.60||curved striae; spectrum; fluorescence<br />
|- <br />
|pink||Cu||1.727-1.740||3.65-3.80<br />
|-<br />
|yellow||Mn||do.||do.||LW/SWUV: green<br />
|-<br />
|emerald green||Mn + Co<sup>3+</sup>||do.||do.<br />
|-<br />
|tourmaline green||Cr<sup>3+</sup>||do.||do.||spectrum<br />
|-<br />
|beryl green||Cr<sup>3+</sup> + Mn||do.||do.||spectrum<br />
|-<br />
|zircon blue||Co<sup>3+</sup> + Cr<sup>3+</sup> + Ti||do.||do.||spectrum<br />
|-<br />
|sapphire blue||Co<sup>3+</sup>||do.||do.||spectrum; fluorescence; CCF<br />
|-<br />
|amethyst violet||Co<sup>3+</sup> + Mn||do.||do.<br />
|-<br />
|alexandrite color change||Cr<sup>3+</sup> + V||do.||do.||spectrum<br />
|-<br />
|lapis lazuli||Co<sup>3+</sup>||1.725||3.52||spectrum; CCF<br />
|-<br />
|colspan="5"|<br />
Strong "tabby" extinction can be seen in all<br /><br />
<nowiki>*</nowiki> Henn, 1995 metions a low value of 1.720.<br />
|}<br />
<br />
===Flux-melt===<br />
<br />
First commercial production of flux-melt synthetic spinel started around 1980, although successful experiments date back to 1848 (by the French chemist Ebelmen). As of 1989 larger volumes of this synthetic appeared on the market, produced in Novosibirsk, Russia.<br />
<br />
Apart from red, blue synthetics have also been produced by the flux-melt process.<br /><br />
These are true synthetics.<br />
<br />
{| {{table}} width="70%" style="margin-left:0;"<br />
|-<br />
|colspan="5" align="center"|Properties of flux-melt synthetic spinel<br />
|-<br />
!color<br />
!coloring agent<br />
!RI<br />
!SG<br />
!other diagnostics<br />
|-<br />
|red||Cr<sup>3+</sup>||1.716-1.719||3.58-3.62||LW/SWUV: distinct red-orange/red<br />flux residu inclusions<br />
|-<br />
|blue||Co<sup>2+</sup> + Fe<sup>2+</sup>||1.719||3.58||spectrum; SWUV: inert; LWUV: weak read<br />flux residu inclusions<br />
|}<br />
Tabby extinction is also seen in these synthetics. Careful observation of inclusions is the main means of separation for red stones. Blue flux-melt synthetics can also be distinguished by the spectrum.<br />
<br />
===Czochralski pulling method===<br />
A recent development (2007) is the procuction of red to pink synthetic spinels by the Czochralski pulling method.<br />
<br />
==Inclusion images==<br />
<br />
[[image:SP713Fzm1brE2U.jpg|thumb|left|240px|Apatite inclusions and their star-like outgrowths along the 60 degree hexagonal plane - looking much like stellate dislocation systems, in a cobalt blue spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
[[image:SP820AO2-16.jpg|thumb|left|240px|Octahedral spinel inclusions with "Saturn-ring" stress fractures in a Burma red spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
[[image:SP820A02colW.jpg|thumb|left|240px|Octahedral inclusions in a Burma red spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
<br clear="all" /><br />
{{inclusions}}<br />
<br />
==Sources==<br />
* ''Introduction to Optical Mineralogy'' 2004 - William D. Nesse ISBN 0195149106<br />
* ''Gemmology 3rd edition'' (2005) - Peter G. Read ISBN 0750664495<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''Edelsteinkuntliches Praktikum'' - Ulrich Henn, Gemmologie Jahrgang 44 / Heft 4 / Dezember 1995, Spinell pp.54-62<br />
* ''Diploma course notes'' 1987 - Gem-A<br />
* ''Über die Eigenshaften von im Flussmittelverfahren hergestellten synthetischen roten und blauen Spinellen aus Russland'' - U. Henn/H. Bank, Gemmologie Jahrgang 41 / Heft 1 / April 1992, pp1-7</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Spinel&diff=7675Spinel2008-09-19T15:58:40Z<p>Doos: /* Refractometer */</p>
<hr />
<div>{{spinel}}<br />
[[Image:Badak1.JPG|left|framed|Badakshan spinel, Afghanistan]] <br clear="left" /><br />
{{images}}<br />
<br />
Spinel is a mineral group. For many centuries, most gem spinels were misidentified as [[sapphire]] or [[ruby]] because they have similar properties and occur in the same geological deposits. The historically significant 5.08 centimeter "Black Prince Ruby" in the center of the British Imperial Crown was only recently identified as a spinel. This stone is irregular in shape and has a somewhat squareish outline. Additionally, it was not faceted, merely polished. Spinels also occur in a vast array of colors. They are slightly softer than sapphires but still very durable.<br>The earliest known use of spinels was as ornaments found in Buddhist tombs in Afghanistan. Blue spinels have been found in England, dating back to the Roman occupation (51 BC to 400 AD).<br />
<br />
<!-- [[Image:DSC_7858_d.jpg|left|thumb|250px|Cobalt Blue Spinel, 3.96 ct <br /> Photo by Jeff Scovil<br />Courtesy of R.W. Wise Goldsmiths]] --><br />
<br clear="all" /> <br />
<br />
==Chemical composition==<br />
<br />
Common spinel belongs to the "spinel series", which belongs to the "spinel group".<br /><br />
The general formula for the spinel group is A<sup>2+</sup>B<sup>3+</sup><sub>2</sub>O<sub>4</sub>. The 3 series of the spinel group are defined by the B<sup>3+</sup> cation.<br /><br />
The spinel group is made up of 3 [[isomorphous replacement|isomorphous]] series.<br />
<br />
The isomorphous series:<br />
* Spinel series (aluminum)<br />
** Spinel - MgAl<sub>2</sub>O<sub>4</sub> (n = 1.719, sg ~ 3.60)<br />
** Hercynite - FeAl<sub>2</sub>O<sub>4</sub><br />
** Gahnite - ZnAl<sub>2</sub>O<sub>4</sub> (n = 1.805, sg = 4.62)<br />
** Galaxite - MnAl<sub>2</sub>O<sub>4</sub><br />
<br />
* Magnetite series (ferric iron)<br />
** Magnetite - FeFe<sub>2</sub>O<sub>4</sub><br />
** Magnesioferrite - MgFe<sub>2</sub>O<sub>4</sub><br />
** Ulvöspinel - FeFeTiO<sub>4</sub><br />
** Franklinite - ZnFe<sub>2</sub>O<sub>4</sub><br />
** Jacobsite - MnFe<sub>2</sub>O<sub>4</sub><br />
** Trevorite - NiFe<sub>2</sub>O<sub>4</sub><br />
<br />
* Chromite series (chrome)<br />
** Chromite - FeCr<sub>2</sub>O<sub>4</sub><br />
** Magnesiochromite - MgCr<sub>2</sub>O<sub>4</sub><br />
<br />
Most of the above series members are rare in nature with the exception of the members of the spinel series, magnetite and chromite. To gemologists common spinel and gahnite are of most interest.<br />
<br />
When gemologist refer to "spinel", we usually imply common spinel, that is the spinel that belongs to the spinel series of the spinel group.<br />
<br />
==Diagnostics==<br />
<br />
Spinel can be confused with many stones by appearance alone, yet optical properties usually rule out most of them.<br /><br />
As spinel belongs to an isomorhous series, the optical and physical properties may vary.<br />
<br />
===Color===<br />
<br />
Spinel: colorless, green, blue, red, black.<br /><br />
Gahnite: blue-green, yellow, brown.<br />
<br />
Varieties:<br />
*Pleonaste (also named ceylonite) - (Mg,Fe)Al<sub>2</sub>0<sub>4</sub> - dark green to blue-green; black<br />
*Gahnospinel - (Mg,Zn)Al<sub>2</sub>0<sub>4</sub> - pale to dark blue<br />
<br />
Spinel is allochromatic and variety colors are produced by transition metals:<br />
* Cr<sup>3+</sup> - red, pink<br />
* Fe<sup>2+</sup> - blue, violet<br />
* Fe<sup>2+</sup> + Co<sup>2+</sup> - darkblue<br />
* Fe<sup>3+</sup> - green<br />
<br />
===Diaphaneity===<br />
<br />
Transparent to opaque.<br />
<br />
===Refractometer===<br />
<br />
Spinel is isotropic and the refractive index of common spinel is generally around 1.712 to 1.720. Red spinel can have a refractive index up to 1.74.<br /><br />
The only other isotropic gemstone that falls within this range is grossular garnet, but it will usually be higher and the color is also different.<br /><br />
Other members of the spinel series, such as gahnite will have higher refractive indices.<br />
<br />
All other gemstones can easily be seperated from spinel by their optic nature.<br />
<br />
Synthetic spinel has a usual refractive index of 1.727.<br /><br />
Gahnospinels have a refractive index between 1.725 and 1.753, while pleonaste (ceylonite) has an R.I. range of 1.77 to 1.80.<br />
Black spinel-hercynite stones from Thailand may have an RI upto 1.789 with an average SG of 3.86.<br />
<br />
===Specific gravity===<br />
<br />
As with the refractive indices, the specif gravity of spinel can vary due to isomorphous replacement.<br /><br />
The values for most gem grade material lies between 3.58 and 3.61. Pleonaste has a S.G. between 3.63 and 3.90.<br /><br />
Gahnospinels may have a specific gravity up to 4.06.<br />
<br />
===Polariscope===<br />
<br />
Common spinel is isotrope and will remain dark under crossed polars.<br /><br />
Verneuil type synthetic spinel will always (maybe with the exception of red) show strong anomalous birefringence due to excess Al<sub>2</sub>O<sub>3</sub> (see [[spinel#synthetics|synthetics]]). This anomalous extinction (as it is currently named) can be seen as "tabby" extinction, resembling the color distribution of a cat's fur, or/and as an Andreas cross caused by pseudo-birefringence.<br />
<br />
Natural and flux-melt synthetic spinel may show weak anomalous extinction.<br />
<br />
===Spectra===<br />
<br />
[[Image:Spectrum natural blue spinel iron.jpg|thumb|left|240px]]<br />Spectrum of a natural blue spinel, colored by iron and minor traces of cobalt.<br clear="all" /><br />
<br />
==Phenomena==<br />
<br />
* Asterism (4 and 6-pointed stars)<br />
* Color change (rare)<br />
<br />
==Occurrence==<br />
<br />
Myanmar; Vietnam; Thailand; Sri Lanka; Pakistan; Afghanistan; Tadshikistan; Kenia; Tanzania; South-Africa; Brazil.<br />
<br />
==Synthetics==<br />
<br />
Spinel is synthesized by the [[flame fusion|Verneuil (flame-fusion) process]] and the flux-melt method, although the first process does not render a true synthetic in most cases.<br />
<br />
===Flame fusion===<br />
<br />
Flame fusion synthetic spinels are produced since 1908 (by accident while creating synthetic corundum), but were not commercially available until 1930.<br />
<br />
It was found that while trying to synthesize spinel through the Verneuil process, the resulting boules would easily fracture and no reasonably sized gemstones could be cut from them.<br /><br />
The ratio MgO to Al<sub>2</sub>O<sub>3</sub> is 1:1 for common spinel and by changing that ratio (adding Al<sub>2</sub>O<sub>3</sub>) the boules became stable. Because that alters the chemical formula of the synthetic, it is not a true synthetic (but accepted as such). Sometimes these are named "beta-corundum" due to the excess of alumnia.<br />
<br />
For the creation of red stones, this alteration was no option and, usually, no red synthetic spinel boules created by the flame-fusion process result in large stones (but are known to excist). The larger sized red synthetic spinels are mostly created with the flux-melt method. The few red synthetics that are created through the Verneuil process will show curved striae like their synthetic corundum cousins.<br />
<br />
As a result in the changing of the MgO:Al<sub>2</sub>O<sub>3</sub> ratio, the flame fusion synthetics have higher refractive indices (usually stable at 1.727) and a higher specific gravity (3.64).<br />
<br />
{| {{table}} width="70%" style="margin-left:0;"<br />
|-<br />
|colspan="5" align="center"|Properties of flame fusion synthetic spinel<br />
|-<br />
!color<br />
!coloring agent<br />
!RI<br />
!SG<br />
!other diagnostics<br />
|-<br />
|colorless||none||1.728-1.740||3.65-3.80||LWUV: green; SWUV: white/blue<br />
|-<br />
|red||Cr<sup>3+</sup>||1.722-1.725*||3.58-3.60||curved striae; spectrum; fluorescence<br />
|- <br />
|pink||Cu||1.727-1.740||3.65-3.80<br />
|-<br />
|yellow||Mn||do.||do.||LW/SWUV: green<br />
|-<br />
|emerald green||Mn + Co<sup>3+</sup>||do.||do.<br />
|-<br />
|tourmaline green||Cr<sup>3+</sup>||do.||do.||spectrum<br />
|-<br />
|beryl green||Cr<sup>3+</sup> + Mn||do.||do.||spectrum<br />
|-<br />
|zircon blue||Co<sup>3+</sup> + Cr<sup>3+</sup> + Ti||do.||do.||spectrum<br />
|-<br />
|sapphire blue||Co<sup>3+</sup>||do.||do.||spectrum; fluorescence; CCF<br />
|-<br />
|amethyst violet||Co<sup>3+</sup> + Mn||do.||do.<br />
|-<br />
|alexandrite color change||Cr<sup>3+</sup> + V||do.||do.||spectrum<br />
|-<br />
|lapis lazuli||Co<sup>3+</sup>||1.725||3.52||spectrum; CCF<br />
|-<br />
|colspan="5"|<br />
Strong "tabby" extinction can be seen in all<br /><br />
<nowiki>*</nowiki> Henn, 1995 metions a low value of 1.720.<br />
|}<br />
<br />
===Flux-melt===<br />
<br />
First commercial production of flux-melt synthetic spinel started around 1980, although successful experiments date back to 1848 (by the French chemist Ebelmen). As of 1989 larger volumes of this synthetic appeared on the market, produced in Novosibirsk, Russia.<br />
<br />
Apart from red, blue synthetics have also been produced by the flux-melt process.<br /><br />
These are true synthetics.<br />
<br />
{| {{table}} width="70%" style="margin-left:0;"<br />
|-<br />
|colspan="5" align="center"|Properties of flux-melt synthetic spinel<br />
|-<br />
!color<br />
!coloring agent<br />
!RI<br />
!SG<br />
!other diagnostics<br />
|-<br />
|red||Cr<sup>3+</sup>||1.716-1.719||3.58-3.62||LW/SWUV: distinct red-orange/red<br />flux residu inclusions<br />
|-<br />
|blue||Co<sup>2+</sup> + Fe<sup>2+</sup>||1.719||3.58||spectrum; SWUV: inert; LWUV: weak read<br />flux residu inclusions<br />
|}<br />
Tabby extinction is also seen in these synthetics. Careful observation of inclusions is the main means of separation for red stones. Blue flux-melt synthetics can also be distinguished by the spectrum.<br />
<br />
===Czochralski pulling method===<br />
A recent development (2007) is the procuction of red to pink synthetic spinels by the Czochralski pulling method.<br />
<br />
==Inclusion images==<br />
<br />
[[image:SP713Fzm1brE2U.jpg|thumb|left|240px|Apatite inclusions and their star-like outgrowths along the 60 degree hexagonal plane - looking much like stellate dislocation systems, in a cobalt blue spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
[[image:SP820AO2-16.jpg|thumb|left|240px|Octahedral spinel inclusions with "Saturn-ring" stress fractures in a Burma red spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
[[image:SP820A02colW.jpg|thumb|left|240px|Octahedral inclusions in a Burma red spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
<br clear="all" /><br />
{{inclusions}}<br />
<br />
==Sources==<br />
* ''Introduction to Optical Mineralogy'' 2004 - William D. Nesse ISBN 0195149106<br />
* ''Gemmology 3rd edition'' (2005) - Peter G. Read ISBN 0750664495<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''Edelsteinkuntliches Praktikum'' - Ulrich Henn, Gemmologie Jahrgang 44 / Heft 4 / Dezember 1995, Spinell pp.54-62<br />
* ''Diploma course notes'' 1987 - Gem-A<br />
* ''Über die Eigenshaften von im Flussmittelverfahren hergestellten synthetischen roten und blauen Spinellen aus Russland'' - U. Henn/H. Bank, Gemmologie Jahrgang 41 / Heft 1 / April 1992, pp1-7</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Topaz&diff=7674Topaz2008-09-19T14:06:32Z<p>Doos: </p>
<hr />
<div>{{Topaz}}<br />
[[Image:Imperial4.jpg|left|thumb|250px|Peach Pink Imperial Topaz<br />Photo courtesy of Lembeck Gems]]<br />
<br clear="left" /><br />
{{images}}<br />
<br />
Topaz is an aluminium silicate mineral with varying amounts of fluorine (F) and hydroxyl (OH) which replace each other through isomorphous replacement. Extreme outer limits with only fluorine or only hydroxyl have not been reported.<br /><br />
As a gemstone, topaz is the birthstone of November and imperial topaz is used to celebrate the 23rd year of marriage.<br />
<br />
<!-- [[Image:Redimper.jpg|left|thumb|250px|Red Imperial Topaz<br />Photo courtesy of Lembeck Gems]]<br clear="left" /> --><br />
<br />
==Diagnostics==<br />
<br />
In color and diaphaneity, topaz can be confused with many gemstones like [[aquamarine]], [[zircon]], [[citrine]], [[peridot]], [[beryl]] and [[sapphire]].<br /><br />
All of these, with the exception of peridot are uniaxial while topaz is biaxial. Peridot is also biaxial, but has higher refractive indices.<br />
<br />
===Color===<br />
<br />
Topaz is allochromatic and occurs in many colors.<br />
* Colorless<br />
* Yellow - colored by color centers<br />
* Green<br />
* Blue - colored by color centers, irradiation/heat treatment<br />
* Red - colored by chromium<br />
* Pink - colored by chromium, heat treatment<br />
* Orange - colored by color centers and chromium<br />
* Brown - colored by color centers<br />
<br />
===Diaphaneity===<br />
<br />
Transparant <br />
<br />
===Refractometer===<br />
<br />
Topaz with high concentrations of fluorine have a lower refractive index (1.61-1.62) than those with high concentrations of hydroxyl (1.63-1.64).<br />
<br />
The optic character of topaz is biaxial with a positive optic sign<br /><br />
Full refractive index range: n<sub>α</sub> = 1.606-1.634, n<sub>β</sub> =1.609-1.637 , n<sub>γ</sub> = 1.616-1.644 with a maximum birefringence of 0.008-0.010 (depending on content of fluorine and hydroxyl).<br />
<br />
Other stones falling in the refractive index range are [[apatite]], [[andalusite]], [[danburite]] and [[tourmaline]].<br />
<br />
===Specific gravity===<br />
<br />
As with the refractive index, the specific gravity changes with high concentrations of hydroxyl and fluorine.<br /><br />
Hydroxyl causes a lower specific gravity (3.53) while fluorine raises the specific gravity of topaz (3.56).<br />
<br />
Topaz sinks in all common heavy liquids while apatite, andalusite, danburite and tourmaline will float in methylene iodide (sg = 3.33).<br />
<br />
===Dichroscope===<br />
<br />
The pleochroism is usually moderate and almost dichroic, except for heated pink stones where it is more profound.<br />
<br />
==Durability==<br />
<br />
Topaz has perfect cleavage in the direction of the basal plane (001), so care should be taken not to knock the gemstone.<br />
<br />
==Phenomena==<br />
<br />
Cat's-eyes.<br />
<br />
==Inclusion images==<br />
<br />
[[image:Q1126HFF4W2.jpg|thumb|left|240px|2-phase inclusions in colorless topaz.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
<br clear="all" /><br />
<br />
{{inclusions}}<br />
<br />
==Treatments==<br />
<br />
Irradiation followed by heat treatment to create blue stones and heat treatment of brownish stones to create pink gemstones.<br /><br />
The irradiation process to create blue stones could make them radioactive and a "cooling down" period is usually taken into account. After that period these stones are perfectly safe.<br />
<br />
==Imitations==<br />
<br />
Recently,2007, yellow-orange-pink flame-fusion corundum is offered in Minas Gerais, Brazil as imperial topaz.<br />
<br />
==Synthetics==<br />
<br />
Although topaz is synthesized, the material is not commercially available for gemstones.<br />
<br />
==Sources==<br />
<br />
* [http://www.gia.edu/newsroom/issue/2798/2696/insider_newsletter_details.cfm#2 From Gems & Gemology: A New Imitation of Imperial Topaz]<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''Gem-A'' Foundation and Diploma notes<br />
* ''Introduction to Optical Mineralogy'' (2004) - William D. Nesse ISBN 0195149106<br />
* ''Gem Reference Guide'' (1995) - GIA ISBN 0873110293<br />
<br />
==External links==<br />
<br />
* [http://www.palagems.com/blue_topaz.htm#son_of_frankenstone Son of Franken-stone: An Update on Irradiated Gemstones] from Pala International</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Spinel&diff=7673Spinel2008-09-17T17:49:03Z<p>Doos: </p>
<hr />
<div>{{spinel}}<br />
[[Image:Badak1.JPG|left|framed|Badakshan spinel, Afghanistan]] <br clear="left" /><br />
{{images}}<br />
<br />
Spinel is a mineral group. For many centuries, most gem spinels were misidentified as [[sapphire]] or [[ruby]] because they have similar properties and occur in the same geological deposits. The historically significant 5.08 centimeter "Black Prince Ruby" in the center of the British Imperial Crown was only recently identified as a spinel. This stone is irregular in shape and has a somewhat squareish outline. Additionally, it was not faceted, merely polished. Spinels also occur in a vast array of colors. They are slightly softer than sapphires but still very durable.<br>The earliest known use of spinels was as ornaments found in Buddhist tombs in Afghanistan. Blue spinels have been found in England, dating back to the Roman occupation (51 BC to 400 AD).<br />
<br />
<!-- [[Image:DSC_7858_d.jpg|left|thumb|250px|Cobalt Blue Spinel, 3.96 ct <br /> Photo by Jeff Scovil<br />Courtesy of R.W. Wise Goldsmiths]] --><br />
<br clear="all" /> <br />
<br />
==Chemical composition==<br />
<br />
Common spinel belongs to the "spinel series", which belongs to the "spinel group".<br /><br />
The general formula for the spinel group is A<sup>2+</sup>B<sup>3+</sup><sub>2</sub>O<sub>4</sub>. The 3 series of the spinel group are defined by the B<sup>3+</sup> cation.<br /><br />
The spinel group is made up of 3 [[isomorphous replacement|isomorphous]] series.<br />
<br />
The isomorphous series:<br />
* Spinel series (aluminum)<br />
** Spinel - MgAl<sub>2</sub>O<sub>4</sub> (n = 1.719, sg ~ 3.60)<br />
** Hercynite - FeAl<sub>2</sub>O<sub>4</sub><br />
** Gahnite - ZnAl<sub>2</sub>O<sub>4</sub> (n = 1.805, sg = 4.62)<br />
** Galaxite - MnAl<sub>2</sub>O<sub>4</sub><br />
<br />
* Magnetite series (ferric iron)<br />
** Magnetite - FeFe<sub>2</sub>O<sub>4</sub><br />
** Magnesioferrite - MgFe<sub>2</sub>O<sub>4</sub><br />
** Ulvöspinel - FeFeTiO<sub>4</sub><br />
** Franklinite - ZnFe<sub>2</sub>O<sub>4</sub><br />
** Jacobsite - MnFe<sub>2</sub>O<sub>4</sub><br />
** Trevorite - NiFe<sub>2</sub>O<sub>4</sub><br />
<br />
* Chromite series (chrome)<br />
** Chromite - FeCr<sub>2</sub>O<sub>4</sub><br />
** Magnesiochromite - MgCr<sub>2</sub>O<sub>4</sub><br />
<br />
Most of the above series members are rare in nature with the exception of the members of the spinel series, magnetite and chromite. To gemologists common spinel and gahnite are of most interest.<br />
<br />
When gemologist refer to "spinel", we usually imply common spinel, that is the spinel that belongs to the spinel series of the spinel group.<br />
<br />
==Diagnostics==<br />
<br />
Spinel can be confused with many stones by appearance alone, yet optical properties usually rule out most of them.<br /><br />
As spinel belongs to an isomorhous series, the optical and physical properties may vary.<br />
<br />
===Color===<br />
<br />
Spinel: colorless, green, blue, red, black.<br /><br />
Gahnite: blue-green, yellow, brown.<br />
<br />
Varieties:<br />
*Pleonaste (also named ceylonite) - (Mg,Fe)Al<sub>2</sub>0<sub>4</sub> - dark green to blue-green; black<br />
*Gahnospinel - (Mg,Zn)Al<sub>2</sub>0<sub>4</sub> - pale to dark blue<br />
<br />
Spinel is allochromatic and variety colors are produced by transition metals:<br />
* Cr<sup>3+</sup> - red, pink<br />
* Fe<sup>2+</sup> - blue, violet<br />
* Fe<sup>2+</sup> + Co<sup>2+</sup> - darkblue<br />
* Fe<sup>3+</sup> - green<br />
<br />
===Diaphaneity===<br />
<br />
Transparent to opaque.<br />
<br />
===Refractometer===<br />
<br />
Spinel is isotropic and the refractive index of common spinel is generally around 1.712 to 1.720. Red spinel can have a refractive index up to 1.74.<br /><br />
The only other isotropic gemstone that falls within this range is grossular garnet, but it will usually be higher and the color is also different.<br /><br />
Other members of the spinel series, such as gahnite will have higher refractive indices.<br />
<br />
All other gemstones can easily be seperated from spinel by their optic nature.<br />
<br />
Synthetic spinel has a usual refractive index of 1.727.<br /><br />
Gahnospinels have a refractive index between 1.725 and 1.753, while pleonaste (ceylonite) has an R.I. range of 1.77 to 1.80.<br />
<br />
===Specific gravity===<br />
<br />
As with the refractive indices, the specif gravity of spinel can vary due to isomorphous replacement.<br /><br />
The values for most gem grade material lies between 3.58 and 3.61. Pleonaste has a S.G. between 3.63 and 3.90.<br /><br />
Gahnospinels may have a specific gravity up to 4.06.<br />
<br />
===Polariscope===<br />
<br />
Common spinel is isotrope and will remain dark under crossed polars.<br /><br />
Verneuil type synthetic spinel will always (maybe with the exception of red) show strong anomalous birefringence due to excess Al<sub>2</sub>O<sub>3</sub> (see [[spinel#synthetics|synthetics]]). This anomalous extinction (as it is currently named) can be seen as "tabby" extinction, resembling the color distribution of a cat's fur, or/and as an Andreas cross caused by pseudo-birefringence.<br />
<br />
Natural and flux-melt synthetic spinel may show weak anomalous extinction.<br />
<br />
===Spectra===<br />
<br />
[[Image:Spectrum natural blue spinel iron.jpg|thumb|left|240px]]<br />Spectrum of a natural blue spinel, colored by iron and minor traces of cobalt.<br clear="all" /><br />
<br />
==Phenomena==<br />
<br />
* Asterism (4 and 6-pointed stars)<br />
* Color change (rare)<br />
<br />
==Occurrence==<br />
<br />
Myanmar; Vietnam; Thailand; Sri Lanka; Pakistan; Afghanistan; Tadshikistan; Kenia; Tanzania; South-Africa; Brazil.<br />
<br />
==Synthetics==<br />
<br />
Spinel is synthesized by the [[flame fusion|Verneuil (flame-fusion) process]] and the flux-melt method, although the first process does not render a true synthetic in most cases.<br />
<br />
===Flame fusion===<br />
<br />
Flame fusion synthetic spinels are produced since 1908 (by accident while creating synthetic corundum), but were not commercially available until 1930.<br />
<br />
It was found that while trying to synthesize spinel through the Verneuil process, the resulting boules would easily fracture and no reasonably sized gemstones could be cut from them.<br /><br />
The ratio MgO to Al<sub>2</sub>O<sub>3</sub> is 1:1 for common spinel and by changing that ratio (adding Al<sub>2</sub>O<sub>3</sub>) the boules became stable. Because that alters the chemical formula of the synthetic, it is not a true synthetic (but accepted as such). Sometimes these are named "beta-corundum" due to the excess of alumnia.<br />
<br />
For the creation of red stones, this alteration was no option and, usually, no red synthetic spinel boules created by the flame-fusion process result in large stones (but are known to excist). The larger sized red synthetic spinels are mostly created with the flux-melt method. The few red synthetics that are created through the Verneuil process will show curved striae like their synthetic corundum cousins.<br />
<br />
As a result in the changing of the MgO:Al<sub>2</sub>O<sub>3</sub> ratio, the flame fusion synthetics have higher refractive indices (usually stable at 1.727) and a higher specific gravity (3.64).<br />
<br />
{| {{table}} width="70%" style="margin-left:0;"<br />
|-<br />
|colspan="5" align="center"|Properties of flame fusion synthetic spinel<br />
|-<br />
!color<br />
!coloring agent<br />
!RI<br />
!SG<br />
!other diagnostics<br />
|-<br />
|colorless||none||1.728-1.740||3.65-3.80||LWUV: green; SWUV: white/blue<br />
|-<br />
|red||Cr<sup>3+</sup>||1.722-1.725*||3.58-3.60||curved striae; spectrum; fluorescence<br />
|- <br />
|pink||Cu||1.727-1.740||3.65-3.80<br />
|-<br />
|yellow||Mn||do.||do.||LW/SWUV: green<br />
|-<br />
|emerald green||Mn + Co<sup>3+</sup>||do.||do.<br />
|-<br />
|tourmaline green||Cr<sup>3+</sup>||do.||do.||spectrum<br />
|-<br />
|beryl green||Cr<sup>3+</sup> + Mn||do.||do.||spectrum<br />
|-<br />
|zircon blue||Co<sup>3+</sup> + Cr<sup>3+</sup> + Ti||do.||do.||spectrum<br />
|-<br />
|sapphire blue||Co<sup>3+</sup>||do.||do.||spectrum; fluorescence; CCF<br />
|-<br />
|amethyst violet||Co<sup>3+</sup> + Mn||do.||do.<br />
|-<br />
|alexandrite color change||Cr<sup>3+</sup> + V||do.||do.||spectrum<br />
|-<br />
|lapis lazuli||Co<sup>3+</sup>||1.725||3.52||spectrum; CCF<br />
|-<br />
|colspan="5"|<br />
Strong "tabby" extinction can be seen in all<br /><br />
<nowiki>*</nowiki> Henn, 1995 metions a low value of 1.720.<br />
|}<br />
<br />
===Flux-melt===<br />
<br />
First commercial production of flux-melt synthetic spinel started around 1980, although successful experiments date back to 1848 (by the French chemist Ebelmen). As of 1989 larger volumes of this synthetic appeared on the market, produced in Novosibirsk, Russia.<br />
<br />
Apart from red, blue synthetics have also been produced by the flux-melt process.<br /><br />
These are true synthetics.<br />
<br />
{| {{table}} width="70%" style="margin-left:0;"<br />
|-<br />
|colspan="5" align="center"|Properties of flux-melt synthetic spinel<br />
|-<br />
!color<br />
!coloring agent<br />
!RI<br />
!SG<br />
!other diagnostics<br />
|-<br />
|red||Cr<sup>3+</sup>||1.716-1.719||3.58-3.62||LW/SWUV: distinct red-orange/red<br />flux residu inclusions<br />
|-<br />
|blue||Co<sup>2+</sup> + Fe<sup>2+</sup>||1.719||3.58||spectrum; SWUV: inert; LWUV: weak read<br />flux residu inclusions<br />
|}<br />
Tabby extinction is also seen in these synthetics. Careful observation of inclusions is the main means of separation for red stones. Blue flux-melt synthetics can also be distinguished by the spectrum.<br />
<br />
===Czochralski pulling method===<br />
A recent development (2007) is the procuction of red to pink synthetic spinels by the Czochralski pulling method.<br />
<br />
==Inclusion images==<br />
<br />
[[image:SP713Fzm1brE2U.jpg|thumb|left|240px|Apatite inclusions and their star-like outgrowths along the 60 degree hexagonal plane - looking much like stellate dislocation systems, in a cobalt blue spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
[[image:SP820AO2-16.jpg|thumb|left|240px|Octahedral spinel inclusions with "Saturn-ring" stress fractures in a Burma red spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
[[image:SP820A02colW.jpg|thumb|left|240px|Octahedral inclusions in a Burma red spinel.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
<br clear="all" /><br />
{{inclusions}}<br />
<br />
==Sources==<br />
* ''Introduction to Optical Mineralogy'' 2004 - William D. Nesse ISBN 0195149106<br />
* ''Gemmology 3rd edition'' (2005) - Peter G. Read ISBN 0750664495<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''Edelsteinkuntliches Praktikum'' - Ulrich Henn, Gemmologie Jahrgang 44 / Heft 4 / Dezember 1995, Spinell pp.54-62<br />
* ''Diploma course notes'' 1987 - Gem-A<br />
* ''Über die Eigenshaften von im Flussmittelverfahren hergestellten synthetischen roten und blauen Spinellen aus Russland'' - U. Henn/H. Bank, Gemmologie Jahrgang 41 / Heft 1 / April 1992, pp1-7</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Topaz&diff=7672Topaz2008-09-17T12:49:16Z<p>Doos: </p>
<hr />
<div>{{Topaz}}<br />
[[Image:Imperial4.jpg|left|thumb|250px|Peach Pink Imperial Topaz<br />Photo courtesy of Lembeck Gems]]<br />
<br clear="left" /><br />
{{images}}<br />
<br />
Topaz is an aluminium silicate mineral with varying amounts of fluorine (F) and hydroxyl (OH) which replace eachother through a type of isomorphous replacement. Extreme outer limits with only fluorine or only hydroxyl have not been reported (so one can not really speak of isomorphous replacement).<br /><br />
As a gemstone, topaz is the birthstone of November and imperial topaz is used to celebrate the 23rd year of marriage.<br />
<br />
<!-- [[Image:Redimper.jpg|left|thumb|250px|Red Imperial Topaz<br />Photo courtesy of Lembeck Gems]]<br clear="left" /> --><br />
<br />
==Diagnostics==<br />
<br />
In color and diaphaneity, topaz can be confused with many gemstones like [[aquamarine]], [[zircon]], [[citrine]], [[peridot]], [[beryl]] and [[sapphire]].<br /><br />
All of these, with the exeption of peridot are uniaxial while topaz is biaxial. Peridot is also biaxial, but has higher refractive indices.<br />
<br />
===Color===<br />
<br />
Topaz is allochromatic and occurs in many colors.<br />
* Colorless<br />
* Yellow - colored by color centers<br />
* Green<br />
* Blue - colored by color centers, irradiation/heat treatment<br />
* Red - colored by chromium<br />
* Pink - colored by chromium, heat treatment<br />
* Orange - colored by color centers and chromium<br />
* Brown - colored by color centers<br />
<br />
===Diaphaneity===<br />
<br />
Transparant <br />
<br />
===Refractometer===<br />
<br />
Topaz with high concentrations of fluorine have a lower refractive index (1.61-1.62) than those with high concentrations of hydroxyl (1.63-1.64).<br />
<br />
The optic charcter of topaz is biaxial with a possitive optic sign<br /><br />
Full refractive index range: n<sub>α</sub> = 1.606-1.634, n<sub>β</sub> =1.609-1.637 , n<sub>γ</sub> = 1.616-1.644 with a maximum birefringence of 0.008-0.010 (depending on content of fluorine and hydroxyl).<br />
<br />
Other stones falling in the refractive index range are [[apatite]], [[andalusite]], [[danburite]] and [[tourmaline]].<br />
<br />
===Specific gravity===<br />
<br />
As with the refractive index, the specific gravity changes with high concentrations of hydroxyl and fluorine.<br /><br />
Hydroxyl causes a lower specific gravity (3.53) while fluorine raises the specific gravity of topaz (3.56).<br />
<br />
Topaz sinks in all common heavy liquids while apatite, andalusite, danburite and tourmaline will float in methylene iodide (sg = 3.33).<br />
<br />
===Dichroscope===<br />
<br />
The pleochroism is usually moderate and almost dichroic, except for heated pink stones where it is more profound.<br />
<br />
==Durability==<br />
<br />
Topaz has perfect cleavage in the direction of the basal plane (001), so care should be taken not to knock the gemstone.<br />
<br />
==Phenomena==<br />
<br />
Cat's-eyes.<br />
<br />
==Inclusion images==<br />
<br />
[[image:Q1126HFF4W2.jpg|thumb|left|240px|2-phase inclusions in coloreless topaz.<br />Photo courtesy of John Huff, gemcollections.com]]<br />
<br clear="all" /><br />
<br />
{{inclusions}}<br />
<br />
==Treatments==<br />
<br />
Irradiation followed by heat treatment to create blue stones and heat treatment of brownish stones to create pink gemstones.<br /><br />
The irradiation process to create blue stones could make them radioactive and a "cooling down" period is usually taken into account. After that period these stones are perfectly safe.<br />
<br />
==Imitations==<br />
<br />
Recently,2007, yellow-orange-pink flame-fusion corundum is offered in Minas Gerais, Brazil as imperial topaz.<br />
<br />
==Synthetics==<br />
<br />
Although topaz is synthesized, the material is not commercially available for gemstones.<br />
<br />
==Sources==<br />
<br />
* [http://www.gia.edu/newsroom/issue/2798/2696/insider_newsletter_details.cfm#2 From Gems & Gemology: A New Imitation of Imperial Topaz]<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''Gem-A'' Foundation and Diploma notes<br />
* ''Introduction to Optical Mineralogy'' (2004) - William D. Nesse ISBN 0195149106<br />
* ''Gem Reference Guide'' (1995) - GIA ISBN 0873110293<br />
<br />
==External links==<br />
<br />
* [http://www.palagems.com/blue_topaz.htm#son_of_frankenstone Son of Franken-stone: An Update on Irradiated Gemstones] from Pala International</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Refractometer&diff=7671Refractometer2008-08-13T16:45:28Z<p>Doos: /* Construction of a gemological refractometer */</p>
<hr />
<div>The refractometer is one of the most important tools in a gemological laboratory. It indicates (not measures) the refraction index of a gemstone, which often gives vital clues to the identity of a gemstone.<br />
<br />
Although one would expect a refractometer to measure the refraction of light inside a gemstone, this is not the case. Instead it is based on a unique optical phenomenon named ''Total Internal Reflection'' (or TIR).<br /><br />
<br />
For a better understanding of the refractometer, you first need to understand [[refraction]].<br />
<br />
==Basic==<br />
<br />
===Construction of a gemological refractometer===<br />
[[image:refractometer.jpg|thumb|400px|right|Cross section of a standard gemological refractometer <br />(modified image from an Eickhorst SR 0.005 refractometer)]]<br />
<br />
<br /><br />
Light (1) enters through the rear of the refractometer through an opening (1a) in (or before) which a yellow sodium filter can be placed. It then hits a mirror (2) which transmits the light to the center of the hemicylinder (3).<br /> This hemicylinder is made of high [[refraction|refractive]] glass (usually N-LaSF by [http://www.schott.com Schott] with a refractive index of ~ 1.88 at n<sub>D</sub> and a hardness of about 6.5 on Moh's scale).<br /> At the boundary between the hemicylinder and the gemstone (4), the light will be partially refracted inside the stone and partially reflected in the hemicylinder (see below on Total Internal reflection). The reflected rays (5) will pass through a reading scale (6) and a lens (7) or a series of lenses, depending on the type of refractometer.<br /><br />
The reflected rays hit a mirror (8) which directs the light to the ocular (9) and then outside the refractometer to your eye (11).<br /><br />
The ocular (9) can slide in and out for better focus and is usually accompanied with a detachable polarizing filter (10).<br />
<br />
As the hemicylinder has a relative low [[hardness]] compared to most gemstones, care must be taken not to scratch it. That would ruin your refractometer, as optical contact between the gemstone and the cylinder would be impossible and would give you false readings.<br />
<br />
<br clear=all><br />
<br />
===Total Internal Reflection===<br />
<br />
[[image:refractometer_diagram.png|frame|left|Inside the refractometer: Total Internal Reflection]]<br />
<br />
When light travels from an optically denser material (with higher index of refraction) to an optically rarer material (with lower index of refraction), all light that reaches the boundary of the two materials will be either reflected inside the denser material or refracted into the rarer material, depending on the angle of incidence of the light.<br />
<br />
For every two media in contact in which light is traveling from the denser to the rarer medium, the dividing line where either the ray of light is totally reflected or refracted is fixed and can be calculated. This dividing line is named the ''critical angle'' (ca). On the left you find an image showing the critical angle as the red line.<br /><br />
When light reaches the boundary of the two materials at an angle larger than this critical angle (the blue line), the ray of light will be totally reflected back into the denser material. Light reaching the boundary at an angle smaller than the critical angle will be refracted out of the denser medium (and a small amount will be reflected) into the rarer medium (the green line). All light traveling precisely on the critical angle will follow the path of the boundary between the two materials.<br />
<br />
N.B.: In this example, the light seems to come from 3 light sources, but the principle is the same when coming from a single point.<br />
<br />
In a hemicylinder, the incident and exiting ray always reach the boundary at a 90 degree angle when directed to the center. Refraction doesn't occur when a light ray is at 90 degrees to the boundary. A hemicylinder is used so there will be no [[refraction]] of the light entering nor leaving the denser material.<br />
<br clear=all /><br />
<br />
The standard gemological refractometer can make use of this phenomenon because the reflected rays of light will appear as a light area on the scale, whilst the refracted rays are not visible (and therefore appear black). The light/dark boundary shown on the scale of the refractometer is a visible representation of the critical angle.<br />
<br /><br />
The standard gemological refractometer thus measures the critical angle between the glass hemi-cylinder and the gemstone and plots that on a calibrated scale. This type of refractometer is hence better named a "critical angle refractometer".<br />
<br />
===Lighting===<br />
<br />
Proper lighting is one of the key features when using the refractometer.<br />
<br />
Although one can get results using a white light source, the standard is monochromatic yellow light with a wavelength of about 589.3nm. This light source is historically used as it was easily produced by burning table salt in a candle (at a very low cost). All gemological refraction indices are based on the use of sodium light (or n<sub>D</sub>). For more information, see [[Fraunhofer]].<br />
<br />
The use of different wavelengths can produce [[dispersion|different readings]]. As the refractive indices of gemstones are measured with an accuracy of 0.001 decimal, sodium light should be used. All gemological tables of refractive indices are produced using this light unless otherwise stated.<br />
<br />
White light may be used for single refractive gemstones or to obtain a first impression. One should look for the boundary between the green and the yellow of the allochromatic white light source.<br />
<br />
However, for double refractive gemstones, one should then switch to a sodium light source, simply for the reason that the double refraction readings in white light may easily overlap and it would be impossible to get a correct reading. And of course the boundary between the lighter and darker areas is better defined, making the reading easier to take.<br />
<br />
Always buy a refractometer with either a sodium filter or a sodium light source.<br />
<br />
===Contact liquids===<br />
<br />
Here things get a bit more complicated.<br />
<br />
Contact liquids are used to create an optical contact between the hemicylinder and the gemstone. This is to prevent air from trapping between the facet of the stone and the hemicylinder, which would ruin the Total Internal Reflection effect.<br />
<br />
As this contact liquid also has it's own refractive index, there will also be Total Internal Reflection between the hemicylinder and the liquid. It is important to ensure that the tiniest drop of liquid is used so the stone doesn't float on the liquid. Use just enough to create a "[[interference|thin film]]". Donald Hoover added to this through personal communication that too much liquid will not only lift up the stone slightly, the reading may also be off slightly due to the refraction inside the liquid (the ray will deviate slightly). With a thin film, this is marginal and will have little to none effect on the reading.<br />
<br />
The result is obviously two Total Internal Reflection readings, one from the hemicylinder-liquid and the other from the liquid-stone boundary (which will be, due to laws of refraction, the same as if no liquid were used). That is the reason you will also see a faint reading near the higher index of the scale on the refractometer, which is the reading of the liquid.<br />
<br />
The refractive index of the liquid sets the limit of which stones can be tested on the refractometer. Usually the liquid has a refractive index of 1.79, but some has a refractive index of 1.81. '''You can not measure stones that have a RI higher than the liquid used.''' Stones with a higher RI than the liquid will give you a "negative reading".<br /><br />
<br />
Liquids with higher RI are available, but they are so toxic that they are only used in specially equipped laboratories. They would, of course, also need a special hemicylinder which will be of higher RI than the liquid.<br />
<br />
You should always shield your contact liquids from light (especially for the 1.81 type) and care should be taken not to let the liquids crystallize.<br />
<br />
The chemical compositions of the liquids are:<br />
* 1.79 - Saturated solution of sulphur and di-idiomethane<br />
* 1.81 - Saturated solution of sulphur, di-idiomethane and tetraidioethylene<br />
<br />
Always wash your hands after you make physical contact with the liquids -- not only for the smell.<br />
<br />
===Use of the refractometer===<br />
<br />
As with every instrument, success depends on proper usage.<br />
<br />
First you apply a very small drop of contact liquid on the center of the hemicylinder of the refractometer, after which you place the stone you want to investigate table down next to the hemicylinder. With your fingernail, slide the stone on the center of the hemicylinder. For an oval stone, place it lengthwise.<br />
<br />
At this point, the contact liquid will suck under the facet and provide an optical contact between the stone and the hemicylinder. Do not apply any pressure to the stone by pushing it down on the cylinder as that would damage the hemicylinder. (Repairs are very costly.) Close the lid of the refractometer to shield the stone from any surrounding light. Remove the polarizing filter if it hasn't been removed already.<br />
<br />
Now, with the light source in place at the back, place your best eye (usually your right one) just before the ocular of the refractometer. You should position your eye so that you look at a straight angle to the ocular, to prevent a "parallax error". The best way to know your eye is in the right position is if you can see the whole scale (or most of it) without moving your eye.<br />
<br />
Now find the dividing line between light and dark on the scale. (For gemstones cut en-cabochon, the technique is slightly different. See the "distant vision" method below.) If the scale seems blurry, you can slide the ocular in and out for better focus. Now you can start taking your readings (explained below).<br />
<br />
When you are finished, gently slide the stone off the hemicylinder and remove the stone with your fingers if possible. It is important to keep the hemicylinder clean, so use a clean cloth or tissue to gently wipe any remaining contact liquid from the cylinder. Do this gently without any pressure, making a North-South motion.<br />
<br />
As mentioned above, the hemicylinder is made of a relatively low hardness glass and can easily scratch. '''So always make sure you keep abrasive materials and sharp objects (like tweezers) away from the hemicylinder.'''<br />
<br />
Look at the images below to see how to properly use the refractometer.<br />
<br />
{| align=center<br />
|+ '''Click images to enlarge'''<br />
|-<br />
| align=center |<br />
<gallery><br />
Image:rf1.jpg |Open liquid bottle and get small drop<br />
Image:rf2.jpg |Carefully place on middle of hemicylinder<br />
Image:rf3.jpg |Drop should be no larger than this!<br />
Image:rf4.jpg |Place stone parallel to length of hemicylinder<br />
</gallery><br />
|}<br />
<br />
N.B: Some people find it hard to get a small drop of liquid directly from the bottle. A different technique is to place a series of small drops (usually 2 or 3) next to the hemicylinder and place the stone on the smallest drop, then slide the stone and liquid together onto the hemicylinder. Alternatively, one can lose excess liquid from the liquid rod by making a few drops next to the hemicylinder and then apply the remainder directly onto the refractometer's hemicylinder. Whichever method one prefers will work.<br />
<br />
[[image:refractometerscale1.jpg |thumb|right|75px|1.544]]<br />
<br />
We notate refractometer readings to a precision of 0.001 (one thousandths). The refractometer scale has subdivision indicators to 0.01 (one hundredths). Between the two horizontal bars which indicate the 0.01, you will need to estimate the final precision.<br />
<br />
In the image on the right, you will see that the shadow edge is between the 1.54 and the 1.55 bars. Between these two values we need to find the last precision. As it is just above the middle, the last precision is 0.004. So the reading is 1.544 .<br />
<br />
Estimating the last decimal needs some practice. Some refractometers, like the Eickhorst ones, have a more detailed division of the scales which makes taking a reading easier. With a little experience, you will find an easier-to-read scale is not needed.<br />
<br clear=all><br />
<br />
====Faceted gemstones====<br />
<br />
Following is the method for taking RI readings that is used for faceted gemstones. En-cabochon and sphere cut gemstones require a somewhat different technique which is explained in the "distant vision" section.<br />
<br />
{| align=center<br />
|-<br />
| align=center |<br />
<gallery><br />
Image:rf5.jpg |Starting position <br /> 1st reading<br />
Image:rf6.jpg |45 degree rotation <br /> 2nd reading<br />
Image:rf7.jpg |90 degree rotation <br /> 3rd reading<br />
Image:rf8.jpg |135 degree rotation <br /> 4th reading<br />
</gallery><br />
|}<br />
<br />
When taking refractometer readings, one usually starts with the largest facet (which is usually the table facet). Place your stone in the starting position, then close the lid of the refractometer. Make sure the light source is on.<br />
<br />
Position your eye in front of the ocular in a way so that it is at a straight angle with the refractometer scale. You will now most likely see a dark region at the top of the scale and a lighter region in the lower part. If you have chosen a monochromatic sodium light source, there will be a sharp line between the lighter and darker areas. That line is named the "shadow edge". (You may also observe 2 less sharp "shadow edges".)<br />
<br />
Place the polarization filter on the ocular and, while looking at the scale, turn the polarizer 90 degrees left and right. You will observe either of two possibilities:<br />
# only one shadow edge is seen <br />
#* the stone is either isotropic or<br />
#* the incident light reaches the stone at an angle parallel to the optic axis and you should turn the stone 90 degrees<br />
# you see the shadow edge move between two values on the scale<br />
#* the stone is uniaxial or<br />
#* the stone is biaxial<br />
<br />
* In the first case, where only one shadow edge is seen, the reading for the shadow edge will remain constant during a 135 degree rotation of the stone. For every rotation reading, take two measurements: one with the polarizing filter in North-South position and one with the polarizing filter in East-West position.<br />
<br />
The readings in the images below indicate a single refractive (isotropic) stone with RI = 1.527, which is most likely glass. (If one finds a single refractive transparent faceted stone with an RI between 1.50 and 1.70, it is most likely glass). Taking four sets of readings (with the polarizer in both positions) on a single refractive stone looks like overkill, which it is; take them anyway.<br /><br />
<br />
<br />
{| <br />
|- <br />
| colspan=2 align=center| '''First reading'''<br />
| colspan=2 align=center| '''Second reading'''<br />
| colspan=2 align=center| '''Third reading'''<br />
| colspan=2 align=center| '''Fourth reading'''<br />
|-<br />
| [[Image:refractometerscale5.jpg |thumb|75px|1.527]]<br />
| [[Image:refractometerscale5.jpg |thumb|75px|1.527]]<br />
| [[Image:refractometerscale5.jpg |thumb|75px|1.527]]<br />
| [[Image:refractometerscale5.jpg |thumb|75px|1.527]]<br />
| [[Image:refractometerscale5.jpg |thumb|75px|1.527]]<br />
| [[Image:refractometerscale5.jpg |thumb|75px|1.527]]<br />
| [[Image:refractometerscale5.jpg |thumb|75px|1.527]]<br />
| [[Image:refractometerscale5.jpg |thumb|75px|1.527]]<br />
|}<br />
<br />
* In the second case, where the shadow edge moves between two values on the scale, write down both values you see, in table form below each other.<br />
<br />
Below are 4 sets of readings of a double refractive stone with a uniaxial optic character (where one reading value remains constant). For every set of readings, you rotate the stone 45 degrees with your fingers without applying pressure while leaving the stone in contact with the hemicylinder.<br />
<br /><br />
<br />
<br />
{| <br />
|- <br />
| colspan=2 align=center| '''First reading'''<br />
| colspan=2 align=center| '''Second reading'''<br />
| colspan=2 align=center| '''Third reading'''<br />
| colspan=2 align=center| '''Fourth reading'''<br />
|-<br />
| [[Image:refractometerscale1.jpg |thumb|75px|1.544 ω]]<br />
| [[Image:refractometerscale2.jpg |thumb|75px|1.553 ε]]<br />
| [[Image:refractometerscale1.jpg |thumb|75px|1.544 ω]]<br />
| [[Image:refractometerscale3.jpg |thumb|75px|1.552 ε]]<br />
| [[Image:refractometerscale1.jpg |thumb|75px|1.544 ω]]<br />
| [[Image:refractometerscale4.jpg |thumb|75px|1.549 ε]]<br />
| [[Image:refractometerscale1.jpg |thumb|75px|1.544 ω]]<br />
| [[Image:refractometerscale3.jpg |thumb|75px|1.552 ε]]<br />
|}<br />
<br />
{| {{table}} align=right style="margin-left:5px;"<br />
|-<br />
!<br />
! 1st<br />
! 2nd<br />
! 3rd<br />
! 4th<br />
|-<br />
| lower readings ω<br />
| 1.544 <br />
| 1.544 <br />
| 1.544 <br />
| 1.544<br />
|-<br />
| higher readings ε<br />
| 1.553 <br />
| 1.552 <br />
| 1.549 <br />
| 1.552<br />
|}<br />
<br />
While taking your refractometer readings, write down the values you read on the scale. For every set of readings, the polarization filter is turned 90 degrees. In addition to this you can also take a fifth reading (180 degree rotation).<br />
<br />
In the example above, the lower readings (1.544) stay constant while the higher readings vary. In other gemstones, the higher value may remain constant while the lower value changes.<br /><br />
:'''Note:''' The lower reading is the reading of lower value, not lower on the scale.<br /><br />
<u>The RI of this stone is 1.544 - 1.553</u> (smallest lower reading and largest higher reading). This indicates quartz.<br />
<br />
To calculate the birefringence of the gemstone being tested, you take the maximum difference between the largest higher reading and the smallest lower reading. In this example, that is 1.553 - 1.544 = 0.009 .<br />
<br />
'''Some gemstones have a lower reading that falls within the range of the refractometer (and the liquid), while the higher reading falls outside the range. Those gemstones will give you just one reading on the refractometer and should not be confused with isotropic gemstones.'''<br />
<br />
* Gemstones may also have two variable lower and higher readings, but the procedure remains the same. You write down the lower and higher readings in a table and calculate the birefringence.<br /><br />
<br />
<br />
{| <br />
|- <br />
| colspan=2 align=center| '''First reading'''<br />
| colspan=2 align=center| '''Second reading'''<br />
| colspan=2 align=center| '''Third reading'''<br />
| colspan=2 align=center| '''Fourth reading'''<br />
|-<br />
| [[Image:refractometerscale6.jpg |thumb|75px|1.613]]<br />
| [[Image:refractometerscale7.jpg |thumb|75px|1.619]]<br />
| [[Image:refractometerscale8.jpg |thumb|75px|1.611 &alpha;]]<br />
| [[Image:refractometerscale10.jpg |thumb|75px|1.616]]<br />
| [[Image:refractometerscale11.jpg |thumb|75px|1.614]]<br />
| [[Image:refractometerscale7.jpg |thumb|75px|1.619]]<br />
| [[Image:refractometerscale8.jpg |thumb|75px|1.611 &alpha;]]<br />
| [[Image:refractometerscale9.jpg |thumb|75px|1.620 &gamma;]]<br />
|}<br />
<br />
{| {{table}} align=right<br />
|-<br />
!<br />
! 1st<br />
! 2nd<br />
! 3rd<br />
! 4th<br />
! difference<br />
|-<br />
| lower readings<br />
| 1.613<br />
| 1.611<br />
| 1.614 <br />
| 1.611<br />
| 0.003<br />
|-<br />
| higher readings<br />
| 1.619 <br />
| 1.616 <br />
| 1.619 <br />
| 1.620<br />
| 0.004<br />
|}<br />
<br />
These readings give an biaxial reading with RI = 1.611-1.620 and a birefringence of 0.009, indicating topaz.<br />
<br clear=all /><br />
You may have noticed some odd looking letters in the image footers, like &alpha;, &gamma;, ε, and &omega; (and &beta; which will be seen later on ). They are not typos but Greek letters whose meanings will become apparent in the discussion on optical sign. You will also learn why we added the "difference" in the biaxial table.<br />
<br />
=====Optical character=====<br />
<br />
Optical character refers to how rays of light travel in gemstones (or most other materials).<br /><br />
In uniaxial and biaxial materials, the incoming light will be polarized in two (uniaxial) or three (biaxial) vibrational directions which all travel at different speeds inside the gemstone. This is due to the molecular packing inside the stone. For a better understanding, we refer to the discussion on [[Double Refraction|double refraction]].<br />
<br />
Gemstones are divided into three categories (characters) depending on the way a ray of light behaves as it passes through the stone:<br />
# isotropic<br />
# uniaxial<br />
# biaxial<br />
<br />
* Isotropic stones are stones in which light travels in all directions at equal speed.<br /><br />
:Among those stones are the ones that form in the cubic system as well as amorphous stones, like glass.<br />
:&bull; On the refractometer you will see one constant reading.<br />
<br />
* Uniaxial means that light travels differently in two directions.<br /><br />
:One ray of light will vibrate in the horizontal plane, which we call the ordinary ray (ω). The other will vibrate in a vertical plane along the c-axis and is called the extra-ordinary ray (ε). This extra-ordinary ray is also the optic axis (the axis along which light behaves as if being isotropic).<br />
:Gemstones that are uniaxial by nature belong to the tetragonal, hexagonal and trigonal crystal systems.<br />
:&bull; You will see one constant and one variable reading on the refractometer.<br />
<br />
* Biaxial gemstones split up incoming light into two rays as well, however the crystallographic directions are labeled as the &alpha;, &gamma; and &beta; rays. The two rays both act as extra-ordinary rays.<br />
:Stones with a biaxial optic character have two optic axes.<br />
:The orthorhombic, monoclinic and triclinic crystal systems are biaxial.<br />
:&bull; This will be shown by two variable readings on the refractometer.<br />
<br />
====Spot readings (distant vision method)====<br />
<br />
[[image:spotreading4.jpg|thumbnail|75px|left]]<br />
<br />
[[image:spotreading2.jpg|thumb|75px]]<br />
<br />
[[image:spotreading3.jpg|thumb|75px]]<br />
<br />
This is the method used to estimate the RI of en-cabochon cut gemstones.<br />
<br />
You place a very small drop of contact liquid on the hemicylinder and place the stone on the drop, on it's most convex side (as in the image on the bottom-right). Remove the polarization filter (if not already done) and close the lid.<br />
<br />
Move your head back about 30 cm from the ocular and look straight to the scale. On the scale, you'll see a reflection of the contact liquid droplet. When you move your head slightly in a "yes-movement", you'll observe the droplet move over the scale. Try to fixate the point where half of the droplet is dark and the other half is bright.<br />
<br />
The image at the top right shows three stages while moving your head. The top droplet is too light and the bottom one is too dark. The one in the center shows a good half dark/half bright droplet.<br />
<br />
Now move your head toward the ocular and estimate the Refractive Index. Unlike with faceted gemstones, we estimate to a 0.01 precision when using this method. The image on the left shows the reflection of the liquid which is half bright/half dark at 1.54. This gemstone may be Amber.<br />
<br />
Alas, one cannot determine birefringence using this method, unless the birefringence is quite large (as with the carbonates). The "birefringence blink" or "carbonate blink" technique makes use of a larger drop of contact liquid and a polarizing plate. As the plate is rotated, the spot will be seen to blink. A crude estimation of birefringence can be made by this technique.<br />
<br />
<br clear=all /><br />
<br />
==Advanced==<br />
<br />
===Optical sign===<br />
<br />
Optic sign in birefringent gemstones is shown as either a plus (+) or a minus (-). The reasons why some stone have a positive sign and others a negative sign lies in the orientation of molecules inside the gemstone. This is explained by the use of an [[indicatrix]] in the [[refraction]] section.<br />
<br />
<u>Isotropic</u> gemstones do not have an optical sign. Light travels at the same speed in all directions.<br />
<br />
<u>Uniaxial</u> stones may have either a positive (+) optical sign or a negative (-) one.<br/ ><br />
We calculate the optic sign by deducting the ordinary ray (ω) from the extra-ordinary ray (ε). So in the case of Quartz with ε = 1.553 and ω = 1.544 that will give us a positive number of 0.009. Hence the optical sign is positive.<br /><br />
A full refractometer result for quartz will therefor be: "RI = 1.553-1.544 uniaxial +" and a birefringence of 0.009.<br />
<br />
'''In uniaxial gemstones the constant reading is always the ordinary ray (ω).'''<br />
<br />
If the ordinary ray is the higher reading in a gemstone (as in the case of Scapolite), there will be a negative optical sign. For instance if you have the following readings: ε = 1.549 and ω = 1.560, the calculation will be 1.549 - 1.560 = -0.011 (so a negative).<br /><br />
This is how we separate Quartz from Scapolite most of the time, the first is uniaxial +, the latter is uniaxial -.<br />
<br />
<u>Biaxial</u> gemstones can also be either positive or negative for the same reasons, however biaxial minerals have three values that correspond with the crystallographic axes. These are the &alpha; (Greek letter alpha), &beta; (Greek letter beta) and &gamma; (Greek letter gamma).<br /><br />
The indicatrix of biaxial materials is somewhat more complex than the uniaxial one.<br /><br />
<br />
In practice, we are not concerned with the intermediate &beta; value, merely with the higher and lower readings we find on the refractometer. As shown previous, we take 4 sets of readings for every orientation of the stone (0 degrees, 45 degrees, 90 degrees and 135 degrees). If we put the readings in a nice table, we can calculate whether the higher or the lower readings vary the most.<br /><br />
<br />
{| {{table}} align=right style="margin-top:3px;"<br />
|-<br />
!<br />
! 1st<br />
! 2nd<br />
! 3rd<br />
! 4th<br />
! difference<br />
|-<br />
| lower readings &alpha;<br />
| 1.613<br />
| 1.611<br />
| 1.614 <br />
| 1.611<br />
| 0.003<br />
|-<br />
| higher readings &gamma;<br />
| 1.619 <br />
| 1.616 <br />
| 1.619 <br />
| 1.620<br />
| 0.004<br />
|}<br />
<br />
As can be seen in the table on the right, the higher readings vary the most (0.004) opposed to the lower readings (0.003), this indicates a positive sign. If the lower reading would have varied the most it would have been biaxial negative.<br /><br />
So for this Topaz the full reading would be: "RI= 1.611-1.620 biaxial +" of course we also mention the birefringence as "DR = 0.009".<br />
<br />
<br />
<br />
<br />
As a word of caution, the explanation above is a crude method as the &beta; value has not been determined. When there is doubt about the identity of the gemstone due to the optic sign, make sure you determine the true value of &beta; (here it could be either 1.614 or 1.616). When the polarizer is used [[Refractometer#Optic_sign|properly]], one will find that true &beta; is at 1.614 for this stone.<br />
<br clear=all /><br />
<br />
===Overview of the crystal systems===<br />
<html><br />
<div style='font-size:10.0pt; font-family:Arial, Helvetica, sans-serif'><br />
<table width="100%" border=0 cellpadding=2 cellspacing=2><br />
<tr><br />
<td width=93 valign=top bgcolor="#E6E6E6"><p><strong>Structure</strong></p></td><br />
<td width=165 valign=top bgcolor="#E6E6E6"><br />
<p><strong>Structure type<br>Crystal axes<br>Angles</strong></p></td><br />
<td width=74 valign=top bgcolor="#E6E6E6"><p><strong>Symmetry <br>(of highest crystal class)</strong></p></td><br />
<td width=106 valign=top bgcolor="#E6E6E6"><p><strong>Optic<br><br />
character</strong></p></td><br />
<td width=70 valign=top bgcolor="#E6E6E6"><p><strong>Refractive index<br>(RI)</strong></p></td><br />
<td width=198 valign=top bgcolor="#E6E6E6"><p><strong>Optic sign</strong></p></td><br />
<td width=97 valign=top bgcolor="#E6E6E6"><p><strong>Pleochroism</strong></p></td><br />
<td valign=top bgcolor="#E6E6E6"><p><strong>Gem <br><br />
examples</strong></p></td><br />
</tr><br />
<tr><br />
<td width=93 valign=top><span>Amorphous</span></td><br />
<td width=165 valign=top><span>No order</span><br />
<p>No axes</p></td><br />
<td width=74 valign=top><span>No symmetry</span></td><br />
<td width=106 valign=top><span>Isotropic</span><br />
<p>Singly refractive</p></td><br />
<td width=70 valign=top><span>1 RI<br><br />
n</span></td><br />
<td width=198 valign=top><span>None</span></td><br />
<td width=97 valign=top><span>None</span></td><br />
<td valign=top><span>Glass</span><br />
<p>Amber</p></td><br />
</tr><br />
<tr><br />
<td width=93 valign=top><span>Cubic</span></td><br />
<td width=165 valign=top>Isometric: 1 axis length<br />
<p>a<sub>1</sub> = a<sub>2</sub> = a<sub>3</sub></p><br />
<p>All at 90°</p></td><br />
<td width=74 valign=top><span>13 planes</span><br />
<p>9 axes</p><br />
<p>Center</p></td><br />
<td width=106 valign=top><span>Isotropic</span><br />
<p>Singly<br />
refractive</p></td><br />
<td width=70 valign=top><span>1 RI<br><br />
n</span></td><br />
<td width=198 valign=top><span>None</span></td><br />
<td width=97 valign=top><span>None</span></td><br />
<td valign=top><span>Diamond</span><br />
<p>Spinel<br></p><br />
<p>Garnet</p></td><br />
</tr><br />
<tr><br />
<td width=93 valign=top><span>Tetragonal</span></td><br />
<td width=165 valign=top><span>Dimetric: 2 axis lengths</span><br />
<p>a<sub>1</sub> = a<sub>2</sub> ≠ c</p><br />
<p>All at 90°</p></td><br />
<td width=74 valign=top><span>5 planes</span><br />
<p>5 axes</p><br />
<p>Center</p></td><br />
<td width=106 valign=top><span>Anisotropic</span><br />
<p>Doubly refractive</p><br />
<p>Uniaxial</p></td><br />
<td width=70 valign=top><span>2 RIs</span><br />
<p>n<sub>w</sub> and <span >n</span><span class="style18" style='font-family:Symbol'><sub>e</sub></span></p></td><br />
<td width=198 valign=top><span>+ =<span > n</span><span class="style18" style='font-family:Symbol'><sub>e </sub></span>&gt;<span > n</span><span class="style18"<br />
style='font-family:Symbol'><sub>w</sub></span></p><br />
<p>&#8211; = <span >n</span><span class="style18" style='font-family:Symbol'><sub>e</sub></span> &lt; <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>w</sub></span></p></td><br />
<td width=97 valign=top><span>May be dichroic</span></td><br />
<td valign=top><span>Zircon</span></td><br />
</tr><br />
<tr><br />
<td width=93 valign=top><span>Hexagonal</span></td><br />
<td width=165 valign=top><span>Dimetric: 2 axis lengths</span><br />
<p>a<sub>1</sub> = a<sub>2</sub> = a<sub>3</sub> ≠ c</p><br />
<p>a axes at 60°;</p><br />
<p>c axis at 90° to their plane</p></td><br />
<td width=74 valign=top><span>7 planes</span><br />
<p>7 axes</p><br />
<p>Center</p></td><br />
<td width=106 valign=top><span>Anisotropic</span><br />
<p>Doubly refractive</p><br />
<p>Uniaxial</p></td><br />
<td width=70 valign=top><span>2 RIs</span><br />
<p>n<sub>w</sub> and <span>n</span><sub>e</sub></p></td><br />
<td width=198 valign=top><p>+ =<span > n</span><span class="style18" style='font-family:Symbol'><sub>e </sub></span>&gt;<span > n</span><span class="style18"<br />
style='font-family:Symbol'><sub>w</sub></span></p><br />
<p>&#8211; = <span >n</span><span class="style18" style='font-family:Symbol'><sub>e</sub></span> &lt; <span >n</span><span class="style18" style='font-family:Symbol'><sub>w</sub></span></p></td><br />
<td width=97 valign=top><span>May be dichroic</span></td><br />
<td valign=top><span>Beryl</span><br />
<p>Apatite</p></td><br />
</tr><br />
<tr><br />
<td width=93 valign=top><span>Trigonal</span></td><br />
<td width=165 valign=top><span>Dimetric: 2 axis lengths</span><br />
<p>a<sub>1</sub> = a<sub>2</sub> = a<sub>3</sub> ≠ c</p><br />
<p>a axes at 60°;</p><br />
<p>c axis at 90° to their plane</p></td><br />
<td width=74 valign=top><span>3 planes</span><br />
<p>4 axes</p><br />
<p>Center</p></td><br />
<td width=106 valign=top><span>Anisotropic</span><br />
<p>Doubly refractive</p><br />
<p>Uniaxial</p></td><br />
<td width=70 valign=top><span>2 RIs</span><br />
<p>n<sub>w</sub> and <span >n</span><span class="style18" style='font-family:Symbol'><sub>e</sub></span></p></td><br />
<td width=198 valign=top><p>+ =<span > n</span><span class="style18" style='font-family:Symbol'><sub>e </sub></span>&gt;<span > n</span><span class="style18" style='font-family:Symbol'><sub>w</sub></span></p><br />
<p>&#8211; = <span >n</span><span class="style18" style='font-family:Symbol'><sub>e</sub></span> &lt; <span >n</span><span class="style18" style='font-family:Symbol'><sub>w</sub></span></p></td><br />
<td width=97 valign=top><span>May be dichroic</span></td><br />
<td valign=top><span>Corundum</span><br />
<p>Quartz</p><br />
<p>Tourmaline</p></td><br />
</tr><br />
<tr><br />
<td width=93 height="109" valign=top><span>Orthorhombic</span></td><br />
<td width=165 valign=top><span>Trimetric: 3 axis lengths</span><br />
<p>a ≠ b ≠ c</p><br />
<p>All at 90°</p></td><br />
<td width=74 valign=top><span>3 planes</span><br />
<p>3 axes</p><br />
<p>Center</p></td><br />
<td width=106 valign=top><span>Anisotropic</span><br />
<p>Doubly refractive</p><br />
<p>Biaxial</p></td><br />
<td width=70 valign=top><span>3 RIs</span><br />
<p><span >n</span><span class="style18" style='font-family:Symbol'><sub>a</sub></span>, <span >n</span><span class="style18" style='font-family:Symbol'><sub>b</sub></span>,<span > n</span><span class="style18" style='font-family:Symbol'><sub>g </sub></span></p></td><br />
<td width=198 valign=top><p>+ = <span >n</span><span class="style18" style='font-family:Symbol'><sub>b</sub></span> closer to <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>a</sub></span></p><br />
<p>&#8211; = <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>b</sub></span> closer to<span > n</span><span class="style18"<br />
style='font-family:Symbol'><sub>g </sub></span></p><br />
<p>± = <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>b</sub></span> midway between <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>a </sub></span>&amp;<span > n</span><span class="style18"<br />
style='font-family:Symbol'><sub>g</sub></span></p></td><br />
<td width=97 valign=top><span>May be trichroic</span></td><br />
<td valign=top><span>Topaz</span><br />
<p>Zoisite </p><br />
<p>Olivine (peridot)</p></td><br />
</tr><br />
<tr><br />
<td width=93 valign=top><span>Monoclinic</span></td><br />
<td width=165 valign=top><span>Trimetric: 3 axis lengths</span><br />
<p>a ≠ b ≠ c</p><br />
<p>2 axes at 90°; </p><br />
<p>1 axis oblique</p></td><br />
<td width=74 valign=top><span>1 axis</span><br />
<p>1 plane</p><br />
<p>Center</p></td><br />
<td width=106 valign=top><span>Anisotropic</span><br />
<p>Doubly refractive</p><br />
<p>Biaxial</p></td><br />
<td width=70 valign=top><span>3 RIs</span><br />
<p><span >n</span><span class="style18" style='font-family:Symbol'><sub>a</sub></span>, <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>b</sub></span>,<span > n</span><span class="style18"<br />
style='font-family:Symbol'><sub>g </sub></span></p></td><br />
<td width=198 valign=top><p>+ = <span >n</span><span class="style18" style='font-family:Symbol'><sub>b</sub></span> closer to <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>a</sub></span></p><br />
<p>&#8211; = <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>b</sub></span> closer to<span > n</span><span class="style18"<br />
style='font-family:Symbol'><sub>g </sub></span></p><br />
<p>± = <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>b</sub></span> midway between <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>a </sub></span>&amp;<span > n</span><span class="style18"<br />
style='font-family:Symbol'><sub>g</sub></span></p></td><br />
<td width=97 valign=top><span>May be trichroic</span></td><br />
<td valign=top><span>Orthoclase</span><br />
<p>Spodumene</p></td><br />
</tr><br />
<tr><br />
<td width=93 valign=top><span>Triclinic</span></td><br />
<td width=165 valign=top><span>Trimetric: 3 axis lengths</span><br />
<p>a ≠ b ≠ c</p><br />
<p>all axes oblique</p></td><br />
<td width=74 valign=top><span>No planes</span><br />
<p>No axes</p><br />
<p>Center</p></td><br />
<td width=106 valign=top><span>Anisotropic</span><br />
<p>Doubly refractive</p><br />
<p>Biaxial</p></td><br />
<td width=70 valign=top><span>3 RIs</span><br />
<p>n<span class="style19"><sub>a</sub></span>, <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>b</sub></span>,<span > n</span><span class="style18"<br />
style='font-family:Symbol'><sub>g </sub></span></p></td><br />
<td width=198 valign=top><p>+ = <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>b</sub></span> closer to <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>a</sub></span></p><br />
<p>&#8211; = <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>b</sub></span> closer to<span > n</span><span class="style18"<br />
style='font-family:Symbol'><sub>g </sub></span></p><br />
<p>± = <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>b</sub></span> midway between <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>a </sub></span>&amp;<span > n</span><span class="style18"<br />
style='font-family:Symbol'><sub>g</sub></span></p></td><br />
<td width=97 valign=top><span>May be trichroic</span></td><br />
<td valign=top><span>Axinite</span><br />
<p>Labradorite</p></td><br />
</tr><br />
</table><br />
</div><br />
<br><br />
</html><br />
<br />
===Optic character/sign with the Refractometer===<br />
<br />
====Optic character/curve variations: Uniaxial or biaxial====<br />
<html><br />
<blockquote><br />
<p>1.<span>&nbsp;&nbsp;&nbsp; </span>Two constant<br />
curves = Uniaxial</p><br />
<p>2.<span>&nbsp;&nbsp;&nbsp; </span>Two variable<br />
curves = Biaxial</p><br />
<p>3.<span>&nbsp;&nbsp;&nbsp; </span>One constant/one<br />
variable which meet = Uniaxial</p><br />
<p>4.<span>&nbsp;&nbsp;&nbsp; </span>One constant/one<br />
variable which don’t meet:</p><br />
<p>&nbsp;&nbsp;&nbsp;&nbsp; Check the polaroid angle of the<br />
constant curve</p><br />
<p>&nbsp;&nbsp;&nbsp;&nbsp; a. Biaxial = polaroid angle of<br />
constant curve = 90°</p><br />
<p>&nbsp;&nbsp;&nbsp;&nbsp; b. Uniaxial = polaroid angle of<br />
constant curve ≠ 90°</p><br />
</blockquote><br />
</html><br />
<br />
====Optic sign====<br />
<html><br />
<p><b>Uniaxial stones</b></p><br />
<blockquote><br />
<p>1.<span>&nbsp;&nbsp;&nbsp; </span>High RI curve<br />
varies = (+)</p><br />
<p>2.<span>&nbsp;&nbsp;&nbsp; </span>Low RI curve<br />
varies = (<span style='font-family:Symbol'>-</span>)</p><br />
<p>3.<span>&nbsp;&nbsp;&nbsp; </span>Both curves<br />
constant: At 0° polaroid angle, only the o-ray is seen</p><br />
<p>&nbsp;&nbsp;&nbsp;&nbsp; a. If low curve is seen = (+)</p><br />
<p>&nbsp;&nbsp;&nbsp;&nbsp; a. If high curve is seen = (<span<br />
style='font-family:Symbol'>-</span>)</p><br />
</blockquote><br />
<p><b>Biaxial stones</b></p><br />
<blockquote><br />
<p>1.<span>&nbsp;&nbsp;&nbsp; </span>If <span<br />
>n</span><span class="style18 style18" style='font-family:Symbol'><sub>b</sub></span> is closer to <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>a</sub></span>, the gem is (+)</p><br />
<p>2.<span>&nbsp;&nbsp;&nbsp; </span>If<span<br />
> n</span><span class="style18 style18" style='font-family:Symbol'><sub>b</sub></span> is closer to <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>g</sub></span>, the gem is (<span<br />
style='font-family:Symbol'>-</span>)</p><br />
<p>3.<span>&nbsp;&nbsp;&nbsp; </span>If <span<br />
>n</span><span class="style18 style18" style='font-family:Symbol'><sub>b</sub></span> is halfway between <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>a</sub></span> and <span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>g</sub></span>, the gem is (±)</p><br />
<p>4.<span>&nbsp;&nbsp;&nbsp; </span>If two possible<br />
betas exist, false beta will have a polaroid angle equal to 90°. True beta will<br />
have a polaroid angle unequal to 90°.</p><br />
</blockquote><br />
</html><br />
<br />
====Polaroid angle====<br />
<html><br />
<ul><br />
<li>0° polaroid angle is when the transmission direction of the<br />
polaroid plate is parallel to the refractometer scale divisions.</li><br />
<li>90° polaroid angle is when the transmission direction of the<br />
polaroid plate is perpendicular to the refractometer scale divisions.</li><br />
</ul><br />
</html><br />
<br />
====Symbols====<br />
<html><br />
<p><b>Uniaxial crystals</b></p><br />
<ul><br />
<li><span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>w</sub></span> = omega, the constant RI of a<br />
uniaxial crystal</li><br />
<li><span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>e</sub></span> = epsilon, the variable RI of a<br />
uniaxial crystal</li><br />
</ul><br />
<p><b>Biaxial crystals</b></p><br />
<ul><br />
<li><span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>a</sub></span> = alpha, the lowest RI of a<br />
biaxial crystal</li><br />
<li><span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>b</sub></span> = beta, the intermediate RI of a<br />
biaxial crystal</li><br />
<li><span >n</span><span class="style18"<br />
style='font-family:Symbol'><sub>g</sub></span> = gamma, the highest RI of a<br />
biaxial crystal</li><br />
</ul><br />
</html><br />
<br />
===Bright line technique===<br />
<br />
In some cases you may find it very hard to get a clear boundary between light and dark using conventional refractometer techniques. In those rare cases you may find it useful to illuminate from the top of the hemicylinder instead of from below.<br />
<br />
Cover up the illumination opening at the rear of the refractometer and open the lid. Place the stone in position as usual and illuminate the stone/hemicylinder in a way that the light is grazing over the surface of the hemicylinder.<br /><br />
This will give you a very bright area when you look through the ocular and/or a very bright line showing the RI value.<br />
This technique is best carried out in a dark environment with a light source that is pointed from the back of the stone (in the direction of the observer). The junction of the stone's facets should be perpendicular to the length axis of the hemicylinder.<br /><br />
With some practice, this will give you a 0.001 precision.<br />
<br />
When allochromatic white light is used, one can determine the relative dispersion of the gemstone aswell as absorption lines in some cases.<br />
<br />
===Kerez effect===<br />
<br />
Some green tourmalines may show upto 8 shadow edges (tourmaline is uniaxial and should only show two shadow edges in one reading). This is to current knowledge due to heat and/or thermal shock while polishing of the table facets.<br /><br />
Little documentation on this subject is at hand.<br />
<br />
Peter Read added the following in personal correspondence:<br /><br />
"The effect in green tourmaline was first reported in 1967 by R. K. Mitchell [ed.: Journal of Gemmology Vol. 10, 194 (1967)] and the name 'Kerez effect' was suggested by him. Work on the effect has since been carried out by Schiffmann and Prof. H. Bank.<br />
In GEMS, the effect first appeared in the 5th edition and was inserted in Chapter 6 (Topaz & Tourmaline) by the late<br />
Robert Kammerling former Director of Identification & Research, GIA Gem Trade Laboratory, USA. .<br />
I understand that the effect is mainly caused by thermal shock due to polishing, and not to chemical constituents."<br />
<br />
<!-- The trick to overcome this is by rotating the polarization filter slightly counterclockwise.<br /> --><br />
Dietrich [1985] mentions that the highest of these readings (lowest on the scale) are the correct ones.<br />
<br />
This phenomenon was named after C.J. Kerez.<br />
<br />
==Different types of refractometers==<br />
<br />
A word of caution to all neophyte gemologists on buying a refractometer. Nowadays inexpensive refractometers are offered on the internet for as low as USD 100.00 . They are mostly fabricated in China and one shouldn't expect too much from them. Especially obtaining a RI for small and en-cabochon cut stones may prove to be difficult. <br /><br />
Some sellers put their own respected company logo on them and pass them on as the best your money can buy.<br /><br />
Always test your new refractometer with a small stone with known refractive index and make sure it is precise at 0.001.<br />
<br />
Although the price is very tempting, a good refractometer is more costly but will last a lifetime when handled with care.<br /><br />
Some of them are outlined below.<br />
<br />
===The Rayner Dialdex refractometer===<br />
<br />
This refractometer differs from most TIR refractometers that it doesn't have an internal scale to read the values from. Instead you will see a "window" with a bright area. By turning a "wheel" on the side of the refractometer, a vertical black band will appear which should be lined up with the lower edge of the bright area. After this one takes the reading from the calibrated wheel.<br /><br />
An external light source should be used.<br />
<br />
===The Duplex refractometer===<br />
<br />
Made in the USA, this refractometer has an extra large window of view. Making it easier to find shadows.<br /><br />
No built-in light source, an external one should be used.<br />
<br />
===The Eickhorst refractometer===<br />
<br />
In contrast to most refractometers, the Eickhorst refractometers have a calibrated scale with 0.005 precision (opposed to the usual 0.01) and this makes estimating the third decimal easier.<br /><br />
Eickhorst also offers gemology modules of great quality and appealing appearance. Some models have an internal light source.<br />
<br />
===The Topcon refractometer===<br />
<br />
This refractometer is made in Japan. Very sturdy metal case and made to last. It is one of the most expensive refractometers on the market.<br /><br />
No internal light source.<br />
<br />
===The Kruess refractometer===<br />
<br />
Kruess is a long established German manufacturer of all sorts of refractometers (not only for gemological purposes). Their line in excellent gemological refractometers includes portable and standard ones, with or without built-in lightning.<br />
<br />
==Related Topics==<br />
* [[Refraction]]<br />
* [[Brilliance]]<br />
* [[Dispersion]]<br />
* [[Interference]]<br />
* [[Immersion cell]]<br />
<br />
==Sources==<br />
* ''Gemmology Third Edition'' - Peter G. Read<br />
* ''Gemology'' - C.S. Hurlbut and G.S.Switzer (1981) Gemology. New York, USA., Wiley, 1st ed., 243 pp.<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th edition - Robert Webster, Anderson<br />
* ''Gem Identification Made Easy'' 3th edition - Bonanno, Antoinette Matlins<br />
* ''Gem-A'' Foundation and Diploma notes<br />
* ''Refraction Anomalies in Tourmalines'' - R. Keith Mitchell, Journal of Gemmology Vol. 10, 194 (1967)<br />
* ''Better refractometer results with the Bright Line technique'' - Dr D.B. Hoover and C. Williams, Journal of Gemmology Vol. 30 No. 5/6, 287-297 (2007)<br />
* ''The Tourmaline Group'' (1985) - Richard Dietrich ISBN 0442218575</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Tourmaline&diff=7670Tourmaline2008-08-08T15:15:30Z<p>Doos: /* Treatments */</p>
<hr />
<div>{{tourmaline}}<br />
[[Image:Bi_Color2.jpg|left|thumb|300px|Bi-color tourmaline<br />Photo courtesy of Lembeck Gems]]<br clear="left" /><br />
Tourmaline is an extremely complex borosilicate that occurs in more than 100 colors. It is hard and durable and very well suited for jewelry. It is a pyroelectric mineral, meaning that when warmed, it attracts dust and other lightweight particles. The Dutch later noticed this property and called the crystals "aschentreckers," and used them to pull ashes out of tobacco pipes. It wasn't introduced into Europe until the early 1700's, when it was imported from the Ceylon by the Dutch. Shortly thereafter it was declared a stone of the Muses, inspiring and enriching the creative processes. It was a talisman for artists, actors and writers. Today, it is mined extensively in South America, East Africa, and in San Diego County, California.<br />
<br />
==Tourmaline group==<br />
<br />
Tourmaline is a large group consisting of complex borosilicates.<br />
<br />
===Species===<br />
Only 5 species of tourmaline are of real importance to gemologists.<br />
*[[Elbaite]]<br />
*[[Liddicoatite]]<br />
*[[Dravite]]<br />
*[[Chromdravite]]<br />
*[[Schorl]]<br />
From the above 5, elbaite is the most important one. Discrimination between elbaite and liddicoatite is usually not attempted.<br />
<br />
===Varieties===<br />
There are many, mainly color, varieties of these species.<br />
<br />
Color varieties (names apply to all species).<br />
*[[Achroite]] - colorless tourmaline<br />
*[[Rubellite]] - red tourmaline (color due to iron and manganese)<br />
*[[Indicolite]] - blue tourmaline (color due to iron)<br />
*[[Verdelite]] - green tourmaline (color due to iron and titanium)<br />
*[[Siberite]] - reddish-violet tourmaline<br />
*[[Watermelon]] - a pink core with green edges<br />
*[[Bi-color]] - two colored tourmaline<br />
*[[Tri-Color]] - three colored tourmaline<br />
<br />
Other varieties.<br />
*[[Paraiba]] - neon colored elbaite tourmaline (color due to copper and manganese)<br />
<br />
==Diagnostics==<br />
<br />
===Color===<br />
<br />
Tourmaline occurs in almost any color. Bi-colored specimens and "watermelons" are common.<br />
<br />
===Refractive index===<br />
<br />
The refractive index of tourmaline lies between 1.610 and 1.698 (usually between 1.62 and 1.64) with a birefringence up to 0.039 (usually 0.019).<br /><br />
n<sub>ω</sub> = 1.631-1.698, n<sub>ε</sub> = 1.610-1.675, optic sign is negative.<br /><br />
The indices of refraction increase with higher iron content.<br />
<br />
Probably due to thermal shock (and/or heat treatment), some stones may show 4 (or even 8) different values per reading. This effect is named the "[[Refractometer#Kerez_effect|Kerez effect]]". Careful recutting of the stone will reveal that it is an outer-edge phenomenom [Dietrich, 1985].<br />
<br />
===Polariscope===<br />
<br />
Some dark colored tourmalines have a so called "closed axis" due to strong selective absorption in the direction of the optic axis and an interference figure may be hard (if not impossible) to find in that case.<br /><br />
Lighter colored stones may be cut with the optic axis perpendicular to the table and good interference figures can be found there.<br />
<br />
Some tourmalines show pseudo-biaxial (due to internal stress) interference figures on lateral rotation with a 2V up to 25° [Nesse, 2004; Dietrich, 1985].<br />
<br />
===Magnification===<br />
<br />
Tourmalines can be of type I to type III clarity grades.<br /><br />
Typical inclusions are:<br />
* Trichites (small thread-like twists)<br />
* Flattened liquid channels running parallel to the optic axis.<br />
* Liquid Veils<br />
* 2 and 3-Phase Inclusions<br />
* Hollow tubes<br />
<br />
[[Image:Tourmaline_veils_tubes.jpg|thumb|left|240px|Tourmaline with liquid veils, hollow tubes and phase inclusions<br />40X Magnification<br /> by Barbra Voltaire]]<br />
[[Image:Tourmaline_xls.jpg|thumb|left|240px|Tourmaline filled with negative crystals, oriented in random directions<br />80X Magnification<br /> by Barbra Voltaire]]<br />
<br clear="left" /><br />
<br />
==Phenomena==<br />
<br />
* Cat's-eyes<br />
<br />
==Treatments==<br />
<br />
Tourmalines may be heat treated to around 700° C to lighten the color, this is a stable alteration.<br /><br />
<br />
A process which seems to work well for deep saturated reds from Nigeria involves slowly ramping the furnace at a rate of 125° C per hour to 520° C, holding the latter temperature for 2 hours and then letting the furnace cool completely.<br /><br />
A deep red Nigerian stone in the gallery below has been heat treated several times as an experiment by Roger Dery with the ramping done last (following instructions by Lisa Elser on the latter part).<br />
<br />
<gallery><br />
Image:RedTourmalineOval5 32ct.jpg|Nigerian red tourmaline. Heat treated at 360° C for 2 hours <br />
Image:RedTourmalineOval5 32v02.jpg|Heat treated at 410° C for 2 hours (same stone)<br />
Image:PinkTourmalineOval5 32ct.jpg|Ramped up to 520° C over a three hour period. Held at 520° C for two hours, followed by cooling (same stone)<br />
</gallery><br />
<br />
<br />
Other treatments are irradiation - for example with cobalt-60 - (stable) and waxing of surface imperfections. The latter treatment is not stable.<br /><br />
Cobalt-60 gamma irradiation gives rise to pink and hot pink colors in some tourmaline.<br />
<br />
==Sources==<br />
<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''The Tourmaline Group'' (1985) - Richard Dietrich ISBN 0442218575<br />
* ''Introduction to Optical mineralogy'' (2004) - William D. Nesse ISBN 0195149106<br />
* ''Refraction Anomalies in Tourmalines'' - R. Keith Mitchell, Journal of Gemmology Vol. 10, 194 (1967)<br />
* [http://gemologyonline.com/Forum/phpBB2/viewtopic.php?t=6350 Red/Pink Tourmaline heat treatment]<br />
* [http://google.com/translate?u=http%3A%2F%2Fwww.embrarad.com.br%2Fgemas.asp&hl=en&ie=UTF8&sl=pt&tl=en Embrarad]</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Tourmaline&diff=7669Tourmaline2008-08-08T15:14:23Z<p>Doos: /* Treatments */</p>
<hr />
<div>{{tourmaline}}<br />
[[Image:Bi_Color2.jpg|left|thumb|300px|Bi-color tourmaline<br />Photo courtesy of Lembeck Gems]]<br clear="left" /><br />
Tourmaline is an extremely complex borosilicate that occurs in more than 100 colors. It is hard and durable and very well suited for jewelry. It is a pyroelectric mineral, meaning that when warmed, it attracts dust and other lightweight particles. The Dutch later noticed this property and called the crystals "aschentreckers," and used them to pull ashes out of tobacco pipes. It wasn't introduced into Europe until the early 1700's, when it was imported from the Ceylon by the Dutch. Shortly thereafter it was declared a stone of the Muses, inspiring and enriching the creative processes. It was a talisman for artists, actors and writers. Today, it is mined extensively in South America, East Africa, and in San Diego County, California.<br />
<br />
==Tourmaline group==<br />
<br />
Tourmaline is a large group consisting of complex borosilicates.<br />
<br />
===Species===<br />
Only 5 species of tourmaline are of real importance to gemologists.<br />
*[[Elbaite]]<br />
*[[Liddicoatite]]<br />
*[[Dravite]]<br />
*[[Chromdravite]]<br />
*[[Schorl]]<br />
From the above 5, elbaite is the most important one. Discrimination between elbaite and liddicoatite is usually not attempted.<br />
<br />
===Varieties===<br />
There are many, mainly color, varieties of these species.<br />
<br />
Color varieties (names apply to all species).<br />
*[[Achroite]] - colorless tourmaline<br />
*[[Rubellite]] - red tourmaline (color due to iron and manganese)<br />
*[[Indicolite]] - blue tourmaline (color due to iron)<br />
*[[Verdelite]] - green tourmaline (color due to iron and titanium)<br />
*[[Siberite]] - reddish-violet tourmaline<br />
*[[Watermelon]] - a pink core with green edges<br />
*[[Bi-color]] - two colored tourmaline<br />
*[[Tri-Color]] - three colored tourmaline<br />
<br />
Other varieties.<br />
*[[Paraiba]] - neon colored elbaite tourmaline (color due to copper and manganese)<br />
<br />
==Diagnostics==<br />
<br />
===Color===<br />
<br />
Tourmaline occurs in almost any color. Bi-colored specimens and "watermelons" are common.<br />
<br />
===Refractive index===<br />
<br />
The refractive index of tourmaline lies between 1.610 and 1.698 (usually between 1.62 and 1.64) with a birefringence up to 0.039 (usually 0.019).<br /><br />
n<sub>ω</sub> = 1.631-1.698, n<sub>ε</sub> = 1.610-1.675, optic sign is negative.<br /><br />
The indices of refraction increase with higher iron content.<br />
<br />
Probably due to thermal shock (and/or heat treatment), some stones may show 4 (or even 8) different values per reading. This effect is named the "[[Refractometer#Kerez_effect|Kerez effect]]". Careful recutting of the stone will reveal that it is an outer-edge phenomenom [Dietrich, 1985].<br />
<br />
===Polariscope===<br />
<br />
Some dark colored tourmalines have a so called "closed axis" due to strong selective absorption in the direction of the optic axis and an interference figure may be hard (if not impossible) to find in that case.<br /><br />
Lighter colored stones may be cut with the optic axis perpendicular to the table and good interference figures can be found there.<br />
<br />
Some tourmalines show pseudo-biaxial (due to internal stress) interference figures on lateral rotation with a 2V up to 25° [Nesse, 2004; Dietrich, 1985].<br />
<br />
===Magnification===<br />
<br />
Tourmalines can be of type I to type III clarity grades.<br /><br />
Typical inclusions are:<br />
* Trichites (small thread-like twists)<br />
* Flattened liquid channels running parallel to the optic axis.<br />
* Liquid Veils<br />
* 2 and 3-Phase Inclusions<br />
* Hollow tubes<br />
<br />
[[Image:Tourmaline_veils_tubes.jpg|thumb|left|240px|Tourmaline with liquid veils, hollow tubes and phase inclusions<br />40X Magnification<br /> by Barbra Voltaire]]<br />
[[Image:Tourmaline_xls.jpg|thumb|left|240px|Tourmaline filled with negative crystals, oriented in random directions<br />80X Magnification<br /> by Barbra Voltaire]]<br />
<br clear="left" /><br />
<br />
==Phenomena==<br />
<br />
* Cat's-eyes<br />
<br />
==Treatments==<br />
<br />
Tourmalines may be heat treated to around 700° C to lighten the color, this is a stable alteration.<br /><br />
<br />
A process which seems to work well for deep saturated reds from Nigeria involves slowly ramping the furnace at a rate of 125° C per hour to 520° C, holding the latter temperature for 2 hours and then letting the furnace cool completely.<br /><br />
A deep red Nigerian stone in the gallery below has been heat treated several times as an experiment by Roger Dery, following instructions by Lisa Elser, with the ramping done last.<br />
<br />
<gallery><br />
Image:RedTourmalineOval5 32ct.jpg|Nigerian red tourmaline. Heat treated at 360° C for 2 hours <br />
Image:RedTourmalineOval5 32v02.jpg|Heat treated at 410° C for 2 hours (same stone)<br />
Image:PinkTourmalineOval5 32ct.jpg|Ramped up to 520° C over a three hour period. Held at 520° C for two hours, followed by cooling (same stone)<br />
</gallery><br />
<br />
<br />
Other treatments are irradiation - for example with cobalt-60 - (stable) and waxing of surface imperfections. The latter treatment is not stable.<br /><br />
Cobalt-60 gamma irradiation gives rise to pink and hot pink colors in some tourmaline.<br />
<br />
==Sources==<br />
<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''The Tourmaline Group'' (1985) - Richard Dietrich ISBN 0442218575<br />
* ''Introduction to Optical mineralogy'' (2004) - William D. Nesse ISBN 0195149106<br />
* ''Refraction Anomalies in Tourmalines'' - R. Keith Mitchell, Journal of Gemmology Vol. 10, 194 (1967)<br />
* [http://gemologyonline.com/Forum/phpBB2/viewtopic.php?t=6350 Red/Pink Tourmaline heat treatment]<br />
* [http://google.com/translate?u=http%3A%2F%2Fwww.embrarad.com.br%2Fgemas.asp&hl=en&ie=UTF8&sl=pt&tl=en Embrarad]</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Tourmaline&diff=7668Tourmaline2008-08-08T15:11:17Z<p>Doos: /* Treatments */</p>
<hr />
<div>{{tourmaline}}<br />
[[Image:Bi_Color2.jpg|left|thumb|300px|Bi-color tourmaline<br />Photo courtesy of Lembeck Gems]]<br clear="left" /><br />
Tourmaline is an extremely complex borosilicate that occurs in more than 100 colors. It is hard and durable and very well suited for jewelry. It is a pyroelectric mineral, meaning that when warmed, it attracts dust and other lightweight particles. The Dutch later noticed this property and called the crystals "aschentreckers," and used them to pull ashes out of tobacco pipes. It wasn't introduced into Europe until the early 1700's, when it was imported from the Ceylon by the Dutch. Shortly thereafter it was declared a stone of the Muses, inspiring and enriching the creative processes. It was a talisman for artists, actors and writers. Today, it is mined extensively in South America, East Africa, and in San Diego County, California.<br />
<br />
==Tourmaline group==<br />
<br />
Tourmaline is a large group consisting of complex borosilicates.<br />
<br />
===Species===<br />
Only 5 species of tourmaline are of real importance to gemologists.<br />
*[[Elbaite]]<br />
*[[Liddicoatite]]<br />
*[[Dravite]]<br />
*[[Chromdravite]]<br />
*[[Schorl]]<br />
From the above 5, elbaite is the most important one. Discrimination between elbaite and liddicoatite is usually not attempted.<br />
<br />
===Varieties===<br />
There are many, mainly color, varieties of these species.<br />
<br />
Color varieties (names apply to all species).<br />
*[[Achroite]] - colorless tourmaline<br />
*[[Rubellite]] - red tourmaline (color due to iron and manganese)<br />
*[[Indicolite]] - blue tourmaline (color due to iron)<br />
*[[Verdelite]] - green tourmaline (color due to iron and titanium)<br />
*[[Siberite]] - reddish-violet tourmaline<br />
*[[Watermelon]] - a pink core with green edges<br />
*[[Bi-color]] - two colored tourmaline<br />
*[[Tri-Color]] - three colored tourmaline<br />
<br />
Other varieties.<br />
*[[Paraiba]] - neon colored elbaite tourmaline (color due to copper and manganese)<br />
<br />
==Diagnostics==<br />
<br />
===Color===<br />
<br />
Tourmaline occurs in almost any color. Bi-colored specimens and "watermelons" are common.<br />
<br />
===Refractive index===<br />
<br />
The refractive index of tourmaline lies between 1.610 and 1.698 (usually between 1.62 and 1.64) with a birefringence up to 0.039 (usually 0.019).<br /><br />
n<sub>ω</sub> = 1.631-1.698, n<sub>ε</sub> = 1.610-1.675, optic sign is negative.<br /><br />
The indices of refraction increase with higher iron content.<br />
<br />
Probably due to thermal shock (and/or heat treatment), some stones may show 4 (or even 8) different values per reading. This effect is named the "[[Refractometer#Kerez_effect|Kerez effect]]". Careful recutting of the stone will reveal that it is an outer-edge phenomenom [Dietrich, 1985].<br />
<br />
===Polariscope===<br />
<br />
Some dark colored tourmalines have a so called "closed axis" due to strong selective absorption in the direction of the optic axis and an interference figure may be hard (if not impossible) to find in that case.<br /><br />
Lighter colored stones may be cut with the optic axis perpendicular to the table and good interference figures can be found there.<br />
<br />
Some tourmalines show pseudo-biaxial (due to internal stress) interference figures on lateral rotation with a 2V up to 25° [Nesse, 2004; Dietrich, 1985].<br />
<br />
===Magnification===<br />
<br />
Tourmalines can be of type I to type III clarity grades.<br /><br />
Typical inclusions are:<br />
* Trichites (small thread-like twists)<br />
* Flattened liquid channels running parallel to the optic axis.<br />
* Liquid Veils<br />
* 2 and 3-Phase Inclusions<br />
* Hollow tubes<br />
<br />
[[Image:Tourmaline_veils_tubes.jpg|thumb|left|240px|Tourmaline with liquid veils, hollow tubes and phase inclusions<br />40X Magnification<br /> by Barbra Voltaire]]<br />
[[Image:Tourmaline_xls.jpg|thumb|left|240px|Tourmaline filled with negative crystals, oriented in random directions<br />80X Magnification<br /> by Barbra Voltaire]]<br />
<br clear="left" /><br />
<br />
==Phenomena==<br />
<br />
* Cat's-eyes<br />
<br />
==Treatments==<br />
<br />
Tourmalines may be heat treated to around 700° C to lighten the color, this is a stable alteration.<br /><br />
<br />
A process which seems to work well for deep saturated reds from Nigeria involves slowly ramping the furnace at a rate of 125° C per hour to 520° C, holding the latter temperature for 2 hours and then letting the furnace cool completely.<br /><br />
A deep red Nigerian stone in the gallery below has been heat treated several times as an experiment by Roger Dery, with the ramping done last.<br />
<br />
<gallery><br />
Image:RedTourmalineOval5 32ct.jpg|Nigerian red tourmaline. Heat treated at 360° C for 2 hours <br />
Image:RedTourmalineOval5 32v02.jpg|Heat treated at 410° C for 2 hours (same stone)<br />
Image:PinkTourmalineOval5 32ct.jpg|Ramped up to 520° C over a three hour period. Held at 520° C for two hours, followed by cooling (same stone)<br />
</gallery><br />
<br />
<br />
Other treatments are irradiation - for example with cobalt-60 - (stable) and waxing of surface imperfections. The latter treatment is not stable.<br /><br />
Cobalt-60 gamma irradiation gives rise to pink and hot pink colors in some tourmaline.<br />
<br />
==Sources==<br />
<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''The Tourmaline Group'' (1985) - Richard Dietrich ISBN 0442218575<br />
* ''Introduction to Optical mineralogy'' (2004) - William D. Nesse ISBN 0195149106<br />
* ''Refraction Anomalies in Tourmalines'' - R. Keith Mitchell, Journal of Gemmology Vol. 10, 194 (1967)<br />
* [http://gemologyonline.com/Forum/phpBB2/viewtopic.php?t=6350 Red/Pink Tourmaline heat treatment]<br />
* [http://google.com/translate?u=http%3A%2F%2Fwww.embrarad.com.br%2Fgemas.asp&hl=en&ie=UTF8&sl=pt&tl=en Embrarad]</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Tourmaline&diff=7667Tourmaline2008-08-08T14:59:42Z<p>Doos: /* Treatments */</p>
<hr />
<div>{{tourmaline}}<br />
[[Image:Bi_Color2.jpg|left|thumb|300px|Bi-color tourmaline<br />Photo courtesy of Lembeck Gems]]<br clear="left" /><br />
Tourmaline is an extremely complex borosilicate that occurs in more than 100 colors. It is hard and durable and very well suited for jewelry. It is a pyroelectric mineral, meaning that when warmed, it attracts dust and other lightweight particles. The Dutch later noticed this property and called the crystals "aschentreckers," and used them to pull ashes out of tobacco pipes. It wasn't introduced into Europe until the early 1700's, when it was imported from the Ceylon by the Dutch. Shortly thereafter it was declared a stone of the Muses, inspiring and enriching the creative processes. It was a talisman for artists, actors and writers. Today, it is mined extensively in South America, East Africa, and in San Diego County, California.<br />
<br />
==Tourmaline group==<br />
<br />
Tourmaline is a large group consisting of complex borosilicates.<br />
<br />
===Species===<br />
Only 5 species of tourmaline are of real importance to gemologists.<br />
*[[Elbaite]]<br />
*[[Liddicoatite]]<br />
*[[Dravite]]<br />
*[[Chromdravite]]<br />
*[[Schorl]]<br />
From the above 5, elbaite is the most important one. Discrimination between elbaite and liddicoatite is usually not attempted.<br />
<br />
===Varieties===<br />
There are many, mainly color, varieties of these species.<br />
<br />
Color varieties (names apply to all species).<br />
*[[Achroite]] - colorless tourmaline<br />
*[[Rubellite]] - red tourmaline (color due to iron and manganese)<br />
*[[Indicolite]] - blue tourmaline (color due to iron)<br />
*[[Verdelite]] - green tourmaline (color due to iron and titanium)<br />
*[[Siberite]] - reddish-violet tourmaline<br />
*[[Watermelon]] - a pink core with green edges<br />
*[[Bi-color]] - two colored tourmaline<br />
*[[Tri-Color]] - three colored tourmaline<br />
<br />
Other varieties.<br />
*[[Paraiba]] - neon colored elbaite tourmaline (color due to copper and manganese)<br />
<br />
==Diagnostics==<br />
<br />
===Color===<br />
<br />
Tourmaline occurs in almost any color. Bi-colored specimens and "watermelons" are common.<br />
<br />
===Refractive index===<br />
<br />
The refractive index of tourmaline lies between 1.610 and 1.698 (usually between 1.62 and 1.64) with a birefringence up to 0.039 (usually 0.019).<br /><br />
n<sub>ω</sub> = 1.631-1.698, n<sub>ε</sub> = 1.610-1.675, optic sign is negative.<br /><br />
The indices of refraction increase with higher iron content.<br />
<br />
Probably due to thermal shock (and/or heat treatment), some stones may show 4 (or even 8) different values per reading. This effect is named the "[[Refractometer#Kerez_effect|Kerez effect]]". Careful recutting of the stone will reveal that it is an outer-edge phenomenom [Dietrich, 1985].<br />
<br />
===Polariscope===<br />
<br />
Some dark colored tourmalines have a so called "closed axis" due to strong selective absorption in the direction of the optic axis and an interference figure may be hard (if not impossible) to find in that case.<br /><br />
Lighter colored stones may be cut with the optic axis perpendicular to the table and good interference figures can be found there.<br />
<br />
Some tourmalines show pseudo-biaxial (due to internal stress) interference figures on lateral rotation with a 2V up to 25° [Nesse, 2004; Dietrich, 1985].<br />
<br />
===Magnification===<br />
<br />
Tourmalines can be of type I to type III clarity grades.<br /><br />
Typical inclusions are:<br />
* Trichites (small thread-like twists)<br />
* Flattened liquid channels running parallel to the optic axis.<br />
* Liquid Veils<br />
* 2 and 3-Phase Inclusions<br />
* Hollow tubes<br />
<br />
[[Image:Tourmaline_veils_tubes.jpg|thumb|left|240px|Tourmaline with liquid veils, hollow tubes and phase inclusions<br />40X Magnification<br /> by Barbra Voltaire]]<br />
[[Image:Tourmaline_xls.jpg|thumb|left|240px|Tourmaline filled with negative crystals, oriented in random directions<br />80X Magnification<br /> by Barbra Voltaire]]<br />
<br clear="left" /><br />
<br />
==Phenomena==<br />
<br />
* Cat's-eyes<br />
<br />
==Treatments==<br />
<br />
Tourmalines may be heat treated to around 700° C to lighten the color, this is a stable alteration.<br /><br />
<br />
A process which seems to work well for deep saturated reds from Nigeria involves slowly ramping the furnace at a rate of 125° C per hour to 520° C, holding the latter temperature for 2 hours and then letting the furnace cool completely.<br /><br />
A deep red Nigerian stone in the gallery below has been heat treated several times as an experiment by Roger Dery, with the ramping done last.<br />
<br />
<gallery><br />
Image:RedTourmalineOval5 32ct.jpg|Nigerian red tourmaline. Heat treated at 360° C for 2 hours <br />
Image:RedTourmalineOval5 32v02.jpg|Heat treated at 410° C for 2 hours (same stone)<br />
Image:PinkTourmalineOval5 32ct.jpg|Ramped up to 520° C over a three hour period. Held at 520° C for two hours, followed by cooling (same stone)<br />
</gallery><br />
<br />
<br />
Other treatments are irradiation - for example with cobalt-60 - (stable) and waxing of surface imperfections. The latter treatment is not stable.<br /><br />
Cobalt-60 irradiation gives rise to pink and hot pink colors in some tourmaline.<br />
<br />
==Sources==<br />
<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''The Tourmaline Group'' (1985) - Richard Dietrich ISBN 0442218575<br />
* ''Introduction to Optical mineralogy'' (2004) - William D. Nesse ISBN 0195149106<br />
* ''Refraction Anomalies in Tourmalines'' - R. Keith Mitchell, Journal of Gemmology Vol. 10, 194 (1967)<br />
* [http://gemologyonline.com/Forum/phpBB2/viewtopic.php?t=6350 Red/Pink Tourmaline heat treatment]<br />
* [http://google.com/translate?u=http%3A%2F%2Fwww.embrarad.com.br%2Fgemas.asp&hl=en&ie=UTF8&sl=pt&tl=en Embrarad]</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Tourmaline&diff=7666Tourmaline2008-08-08T14:56:22Z<p>Doos: /* Treatments */</p>
<hr />
<div>{{tourmaline}}<br />
[[Image:Bi_Color2.jpg|left|thumb|300px|Bi-color tourmaline<br />Photo courtesy of Lembeck Gems]]<br clear="left" /><br />
Tourmaline is an extremely complex borosilicate that occurs in more than 100 colors. It is hard and durable and very well suited for jewelry. It is a pyroelectric mineral, meaning that when warmed, it attracts dust and other lightweight particles. The Dutch later noticed this property and called the crystals "aschentreckers," and used them to pull ashes out of tobacco pipes. It wasn't introduced into Europe until the early 1700's, when it was imported from the Ceylon by the Dutch. Shortly thereafter it was declared a stone of the Muses, inspiring and enriching the creative processes. It was a talisman for artists, actors and writers. Today, it is mined extensively in South America, East Africa, and in San Diego County, California.<br />
<br />
==Tourmaline group==<br />
<br />
Tourmaline is a large group consisting of complex borosilicates.<br />
<br />
===Species===<br />
Only 5 species of tourmaline are of real importance to gemologists.<br />
*[[Elbaite]]<br />
*[[Liddicoatite]]<br />
*[[Dravite]]<br />
*[[Chromdravite]]<br />
*[[Schorl]]<br />
From the above 5, elbaite is the most important one. Discrimination between elbaite and liddicoatite is usually not attempted.<br />
<br />
===Varieties===<br />
There are many, mainly color, varieties of these species.<br />
<br />
Color varieties (names apply to all species).<br />
*[[Achroite]] - colorless tourmaline<br />
*[[Rubellite]] - red tourmaline (color due to iron and manganese)<br />
*[[Indicolite]] - blue tourmaline (color due to iron)<br />
*[[Verdelite]] - green tourmaline (color due to iron and titanium)<br />
*[[Siberite]] - reddish-violet tourmaline<br />
*[[Watermelon]] - a pink core with green edges<br />
*[[Bi-color]] - two colored tourmaline<br />
*[[Tri-Color]] - three colored tourmaline<br />
<br />
Other varieties.<br />
*[[Paraiba]] - neon colored elbaite tourmaline (color due to copper and manganese)<br />
<br />
==Diagnostics==<br />
<br />
===Color===<br />
<br />
Tourmaline occurs in almost any color. Bi-colored specimens and "watermelons" are common.<br />
<br />
===Refractive index===<br />
<br />
The refractive index of tourmaline lies between 1.610 and 1.698 (usually between 1.62 and 1.64) with a birefringence up to 0.039 (usually 0.019).<br /><br />
n<sub>ω</sub> = 1.631-1.698, n<sub>ε</sub> = 1.610-1.675, optic sign is negative.<br /><br />
The indices of refraction increase with higher iron content.<br />
<br />
Probably due to thermal shock (and/or heat treatment), some stones may show 4 (or even 8) different values per reading. This effect is named the "[[Refractometer#Kerez_effect|Kerez effect]]". Careful recutting of the stone will reveal that it is an outer-edge phenomenom [Dietrich, 1985].<br />
<br />
===Polariscope===<br />
<br />
Some dark colored tourmalines have a so called "closed axis" due to strong selective absorption in the direction of the optic axis and an interference figure may be hard (if not impossible) to find in that case.<br /><br />
Lighter colored stones may be cut with the optic axis perpendicular to the table and good interference figures can be found there.<br />
<br />
Some tourmalines show pseudo-biaxial (due to internal stress) interference figures on lateral rotation with a 2V up to 25° [Nesse, 2004; Dietrich, 1985].<br />
<br />
===Magnification===<br />
<br />
Tourmalines can be of type I to type III clarity grades.<br /><br />
Typical inclusions are:<br />
* Trichites (small thread-like twists)<br />
* Flattened liquid channels running parallel to the optic axis.<br />
* Liquid Veils<br />
* 2 and 3-Phase Inclusions<br />
* Hollow tubes<br />
<br />
[[Image:Tourmaline_veils_tubes.jpg|thumb|left|240px|Tourmaline with liquid veils, hollow tubes and phase inclusions<br />40X Magnification<br /> by Barbra Voltaire]]<br />
[[Image:Tourmaline_xls.jpg|thumb|left|240px|Tourmaline filled with negative crystals, oriented in random directions<br />80X Magnification<br /> by Barbra Voltaire]]<br />
<br clear="left" /><br />
<br />
==Phenomena==<br />
<br />
* Cat's-eyes<br />
<br />
==Treatments==<br />
<br />
Tourmalines may be heat treated to around 700° C to lighten the color, this is a stable alteration.<br /><br />
<br />
A process which seems to work well for deep saturated reds from Nigeria involves slowly ramping the furnace at a rate of 125° C per hour to 520° C, holding the latter temperature for 2 hours and then letting the furnace cool completely.<br /><br />
A deep red Nigerian stone in the gallery below has been heat treated several times as an experiment by Roger Dery, with the ramping done last.<br />
<br />
<gallery><br />
Image:RedTourmalineOval5 32ct.jpg|Nigerian red tourmaline. Heat treated at 360° C for 2 hours <br />
Image:RedTourmalineOval5 32v02.jpg|Heat treated at 410° C for 2 hours (same stone)<br />
Image:PinkTourmalineOval5 32ct.jpg|Ramped up to 520° C over a three hour period. Held at 520° C for two hours, followed by cooling (same stone)<br />
</gallery><br />
<br />
<br />
Other treatments are irradiation - for example with cobalt-60 - (stable) and waxing of surface imperfections. The latter treatment is not stable.<br /><br />
Cobalt-60 irradiation gives to rise pink and hot pink colors in tourmaline.<br />
<br />
==Sources==<br />
<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''The Tourmaline Group'' (1985) - Richard Dietrich ISBN 0442218575<br />
* ''Introduction to Optical mineralogy'' (2004) - William D. Nesse ISBN 0195149106<br />
* ''Refraction Anomalies in Tourmalines'' - R. Keith Mitchell, Journal of Gemmology Vol. 10, 194 (1967)<br />
* [http://gemologyonline.com/Forum/phpBB2/viewtopic.php?t=6350 Red/Pink Tourmaline heat treatment]<br />
* [http://google.com/translate?u=http%3A%2F%2Fwww.embrarad.com.br%2Fgemas.asp&hl=en&ie=UTF8&sl=pt&tl=en Embrarad]</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Tourmaline&diff=7665Tourmaline2008-08-08T14:55:46Z<p>Doos: /* Sources */</p>
<hr />
<div>{{tourmaline}}<br />
[[Image:Bi_Color2.jpg|left|thumb|300px|Bi-color tourmaline<br />Photo courtesy of Lembeck Gems]]<br clear="left" /><br />
Tourmaline is an extremely complex borosilicate that occurs in more than 100 colors. It is hard and durable and very well suited for jewelry. It is a pyroelectric mineral, meaning that when warmed, it attracts dust and other lightweight particles. The Dutch later noticed this property and called the crystals "aschentreckers," and used them to pull ashes out of tobacco pipes. It wasn't introduced into Europe until the early 1700's, when it was imported from the Ceylon by the Dutch. Shortly thereafter it was declared a stone of the Muses, inspiring and enriching the creative processes. It was a talisman for artists, actors and writers. Today, it is mined extensively in South America, East Africa, and in San Diego County, California.<br />
<br />
==Tourmaline group==<br />
<br />
Tourmaline is a large group consisting of complex borosilicates.<br />
<br />
===Species===<br />
Only 5 species of tourmaline are of real importance to gemologists.<br />
*[[Elbaite]]<br />
*[[Liddicoatite]]<br />
*[[Dravite]]<br />
*[[Chromdravite]]<br />
*[[Schorl]]<br />
From the above 5, elbaite is the most important one. Discrimination between elbaite and liddicoatite is usually not attempted.<br />
<br />
===Varieties===<br />
There are many, mainly color, varieties of these species.<br />
<br />
Color varieties (names apply to all species).<br />
*[[Achroite]] - colorless tourmaline<br />
*[[Rubellite]] - red tourmaline (color due to iron and manganese)<br />
*[[Indicolite]] - blue tourmaline (color due to iron)<br />
*[[Verdelite]] - green tourmaline (color due to iron and titanium)<br />
*[[Siberite]] - reddish-violet tourmaline<br />
*[[Watermelon]] - a pink core with green edges<br />
*[[Bi-color]] - two colored tourmaline<br />
*[[Tri-Color]] - three colored tourmaline<br />
<br />
Other varieties.<br />
*[[Paraiba]] - neon colored elbaite tourmaline (color due to copper and manganese)<br />
<br />
==Diagnostics==<br />
<br />
===Color===<br />
<br />
Tourmaline occurs in almost any color. Bi-colored specimens and "watermelons" are common.<br />
<br />
===Refractive index===<br />
<br />
The refractive index of tourmaline lies between 1.610 and 1.698 (usually between 1.62 and 1.64) with a birefringence up to 0.039 (usually 0.019).<br /><br />
n<sub>ω</sub> = 1.631-1.698, n<sub>ε</sub> = 1.610-1.675, optic sign is negative.<br /><br />
The indices of refraction increase with higher iron content.<br />
<br />
Probably due to thermal shock (and/or heat treatment), some stones may show 4 (or even 8) different values per reading. This effect is named the "[[Refractometer#Kerez_effect|Kerez effect]]". Careful recutting of the stone will reveal that it is an outer-edge phenomenom [Dietrich, 1985].<br />
<br />
===Polariscope===<br />
<br />
Some dark colored tourmalines have a so called "closed axis" due to strong selective absorption in the direction of the optic axis and an interference figure may be hard (if not impossible) to find in that case.<br /><br />
Lighter colored stones may be cut with the optic axis perpendicular to the table and good interference figures can be found there.<br />
<br />
Some tourmalines show pseudo-biaxial (due to internal stress) interference figures on lateral rotation with a 2V up to 25° [Nesse, 2004; Dietrich, 1985].<br />
<br />
===Magnification===<br />
<br />
Tourmalines can be of type I to type III clarity grades.<br /><br />
Typical inclusions are:<br />
* Trichites (small thread-like twists)<br />
* Flattened liquid channels running parallel to the optic axis.<br />
* Liquid Veils<br />
* 2 and 3-Phase Inclusions<br />
* Hollow tubes<br />
<br />
[[Image:Tourmaline_veils_tubes.jpg|thumb|left|240px|Tourmaline with liquid veils, hollow tubes and phase inclusions<br />40X Magnification<br /> by Barbra Voltaire]]<br />
[[Image:Tourmaline_xls.jpg|thumb|left|240px|Tourmaline filled with negative crystals, oriented in random directions<br />80X Magnification<br /> by Barbra Voltaire]]<br />
<br clear="left" /><br />
<br />
==Phenomena==<br />
<br />
* Cat's-eyes<br />
<br />
==Treatments==<br />
<br />
Tourmalines may be heat treated to around 700° C to lighten the color, this is a stable alteration.<br /><br />
<br />
A process which seems to work well for deep saturated reds from Nigeria involves slowly ramping the furnace at a rate of 125° C per hour to 520° C, holding the latter temperature for 2 hours and then letting the furnace cool completely.<br /><br />
A deep red Nigerian stone in the gallery below has been heat treated several times as an experiment by Roger Dery, with the ramping done last.<br />
<br />
<gallery><br />
Image:RedTourmalineOval5 32ct.jpg|Nigerian red tourmaline. Heat treated at 360° C for 2 hours <br />
Image:RedTourmalineOval5 32v02.jpg|Nigerian red tourmaline. Heat treated at 410° C for 2 hours (same stone)<br />
Image:PinkTourmalineOval5 32ct.jpg|ramped up to 520° C over a three hour period. Held at 520° C for two hours, followed by cooling (same stone)<br />
</gallery><br />
<br />
<br />
Other treatments are irradiation - for example with cobalt-60 - (stable) and waxing of surface imperfections. The latter treatment is not stable.<br /><br />
Cobalt-60 irradiation gives to rise pink and hot pink colors in tourmaline.<br />
<br />
==Sources==<br />
<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''The Tourmaline Group'' (1985) - Richard Dietrich ISBN 0442218575<br />
* ''Introduction to Optical mineralogy'' (2004) - William D. Nesse ISBN 0195149106<br />
* ''Refraction Anomalies in Tourmalines'' - R. Keith Mitchell, Journal of Gemmology Vol. 10, 194 (1967)<br />
* [http://gemologyonline.com/Forum/phpBB2/viewtopic.php?t=6350 Red/Pink Tourmaline heat treatment]<br />
* [http://google.com/translate?u=http%3A%2F%2Fwww.embrarad.com.br%2Fgemas.asp&hl=en&ie=UTF8&sl=pt&tl=en Embrarad]</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Tourmaline&diff=7664Tourmaline2008-08-08T14:54:07Z<p>Doos: /* Treatments */</p>
<hr />
<div>{{tourmaline}}<br />
[[Image:Bi_Color2.jpg|left|thumb|300px|Bi-color tourmaline<br />Photo courtesy of Lembeck Gems]]<br clear="left" /><br />
Tourmaline is an extremely complex borosilicate that occurs in more than 100 colors. It is hard and durable and very well suited for jewelry. It is a pyroelectric mineral, meaning that when warmed, it attracts dust and other lightweight particles. The Dutch later noticed this property and called the crystals "aschentreckers," and used them to pull ashes out of tobacco pipes. It wasn't introduced into Europe until the early 1700's, when it was imported from the Ceylon by the Dutch. Shortly thereafter it was declared a stone of the Muses, inspiring and enriching the creative processes. It was a talisman for artists, actors and writers. Today, it is mined extensively in South America, East Africa, and in San Diego County, California.<br />
<br />
==Tourmaline group==<br />
<br />
Tourmaline is a large group consisting of complex borosilicates.<br />
<br />
===Species===<br />
Only 5 species of tourmaline are of real importance to gemologists.<br />
*[[Elbaite]]<br />
*[[Liddicoatite]]<br />
*[[Dravite]]<br />
*[[Chromdravite]]<br />
*[[Schorl]]<br />
From the above 5, elbaite is the most important one. Discrimination between elbaite and liddicoatite is usually not attempted.<br />
<br />
===Varieties===<br />
There are many, mainly color, varieties of these species.<br />
<br />
Color varieties (names apply to all species).<br />
*[[Achroite]] - colorless tourmaline<br />
*[[Rubellite]] - red tourmaline (color due to iron and manganese)<br />
*[[Indicolite]] - blue tourmaline (color due to iron)<br />
*[[Verdelite]] - green tourmaline (color due to iron and titanium)<br />
*[[Siberite]] - reddish-violet tourmaline<br />
*[[Watermelon]] - a pink core with green edges<br />
*[[Bi-color]] - two colored tourmaline<br />
*[[Tri-Color]] - three colored tourmaline<br />
<br />
Other varieties.<br />
*[[Paraiba]] - neon colored elbaite tourmaline (color due to copper and manganese)<br />
<br />
==Diagnostics==<br />
<br />
===Color===<br />
<br />
Tourmaline occurs in almost any color. Bi-colored specimens and "watermelons" are common.<br />
<br />
===Refractive index===<br />
<br />
The refractive index of tourmaline lies between 1.610 and 1.698 (usually between 1.62 and 1.64) with a birefringence up to 0.039 (usually 0.019).<br /><br />
n<sub>ω</sub> = 1.631-1.698, n<sub>ε</sub> = 1.610-1.675, optic sign is negative.<br /><br />
The indices of refraction increase with higher iron content.<br />
<br />
Probably due to thermal shock (and/or heat treatment), some stones may show 4 (or even 8) different values per reading. This effect is named the "[[Refractometer#Kerez_effect|Kerez effect]]". Careful recutting of the stone will reveal that it is an outer-edge phenomenom [Dietrich, 1985].<br />
<br />
===Polariscope===<br />
<br />
Some dark colored tourmalines have a so called "closed axis" due to strong selective absorption in the direction of the optic axis and an interference figure may be hard (if not impossible) to find in that case.<br /><br />
Lighter colored stones may be cut with the optic axis perpendicular to the table and good interference figures can be found there.<br />
<br />
Some tourmalines show pseudo-biaxial (due to internal stress) interference figures on lateral rotation with a 2V up to 25° [Nesse, 2004; Dietrich, 1985].<br />
<br />
===Magnification===<br />
<br />
Tourmalines can be of type I to type III clarity grades.<br /><br />
Typical inclusions are:<br />
* Trichites (small thread-like twists)<br />
* Flattened liquid channels running parallel to the optic axis.<br />
* Liquid Veils<br />
* 2 and 3-Phase Inclusions<br />
* Hollow tubes<br />
<br />
[[Image:Tourmaline_veils_tubes.jpg|thumb|left|240px|Tourmaline with liquid veils, hollow tubes and phase inclusions<br />40X Magnification<br /> by Barbra Voltaire]]<br />
[[Image:Tourmaline_xls.jpg|thumb|left|240px|Tourmaline filled with negative crystals, oriented in random directions<br />80X Magnification<br /> by Barbra Voltaire]]<br />
<br clear="left" /><br />
<br />
==Phenomena==<br />
<br />
* Cat's-eyes<br />
<br />
==Treatments==<br />
<br />
Tourmalines may be heat treated to around 700° C to lighten the color, this is a stable alteration.<br /><br />
<br />
A process which seems to work well for deep saturated reds from Nigeria involves slowly ramping the furnace at a rate of 125° C per hour to 520° C, holding the latter temperature for 2 hours and then letting the furnace cool completely.<br /><br />
A deep red Nigerian stone in the gallery below has been heat treated several times as an experiment by Roger Dery, with the ramping done last.<br />
<br />
<gallery><br />
Image:RedTourmalineOval5 32ct.jpg|Nigerian red tourmaline. Heat treated at 360° C for 2 hours <br />
Image:RedTourmalineOval5 32v02.jpg|Nigerian red tourmaline. Heat treated at 410° C for 2 hours (same stone)<br />
Image:PinkTourmalineOval5 32ct.jpg|ramped up to 520° C over a three hour period. Held at 520° C for two hours, followed by cooling (same stone)<br />
</gallery><br />
<br />
<br />
Other treatments are irradiation - for example with cobalt-60 - (stable) and waxing of surface imperfections. The latter treatment is not stable.<br /><br />
Cobalt-60 irradiation gives to rise pink and hot pink colors in tourmaline.<br />
<br />
==Sources==<br />
<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''The Tourmaline Group'' (1985) - Richard Dietrich ISBN 0442218575<br />
* ''Introduction to Optical mineralogy'' (2004) - William D. Nesse ISBN 0195149106<br />
* ''Refraction Anomalies in Tourmalines'' - R. Keith Mitchell, Journal of Gemmology Vol. 10, 194 (1967)</div>Dooshttp://www.gemologyproject.com/wiki/index.php?title=Tourmaline&diff=7663Tourmaline2008-08-08T14:53:33Z<p>Doos: /* Treatments */</p>
<hr />
<div>{{tourmaline}}<br />
[[Image:Bi_Color2.jpg|left|thumb|300px|Bi-color tourmaline<br />Photo courtesy of Lembeck Gems]]<br clear="left" /><br />
Tourmaline is an extremely complex borosilicate that occurs in more than 100 colors. It is hard and durable and very well suited for jewelry. It is a pyroelectric mineral, meaning that when warmed, it attracts dust and other lightweight particles. The Dutch later noticed this property and called the crystals "aschentreckers," and used them to pull ashes out of tobacco pipes. It wasn't introduced into Europe until the early 1700's, when it was imported from the Ceylon by the Dutch. Shortly thereafter it was declared a stone of the Muses, inspiring and enriching the creative processes. It was a talisman for artists, actors and writers. Today, it is mined extensively in South America, East Africa, and in San Diego County, California.<br />
<br />
==Tourmaline group==<br />
<br />
Tourmaline is a large group consisting of complex borosilicates.<br />
<br />
===Species===<br />
Only 5 species of tourmaline are of real importance to gemologists.<br />
*[[Elbaite]]<br />
*[[Liddicoatite]]<br />
*[[Dravite]]<br />
*[[Chromdravite]]<br />
*[[Schorl]]<br />
From the above 5, elbaite is the most important one. Discrimination between elbaite and liddicoatite is usually not attempted.<br />
<br />
===Varieties===<br />
There are many, mainly color, varieties of these species.<br />
<br />
Color varieties (names apply to all species).<br />
*[[Achroite]] - colorless tourmaline<br />
*[[Rubellite]] - red tourmaline (color due to iron and manganese)<br />
*[[Indicolite]] - blue tourmaline (color due to iron)<br />
*[[Verdelite]] - green tourmaline (color due to iron and titanium)<br />
*[[Siberite]] - reddish-violet tourmaline<br />
*[[Watermelon]] - a pink core with green edges<br />
*[[Bi-color]] - two colored tourmaline<br />
*[[Tri-Color]] - three colored tourmaline<br />
<br />
Other varieties.<br />
*[[Paraiba]] - neon colored elbaite tourmaline (color due to copper and manganese)<br />
<br />
==Diagnostics==<br />
<br />
===Color===<br />
<br />
Tourmaline occurs in almost any color. Bi-colored specimens and "watermelons" are common.<br />
<br />
===Refractive index===<br />
<br />
The refractive index of tourmaline lies between 1.610 and 1.698 (usually between 1.62 and 1.64) with a birefringence up to 0.039 (usually 0.019).<br /><br />
n<sub>ω</sub> = 1.631-1.698, n<sub>ε</sub> = 1.610-1.675, optic sign is negative.<br /><br />
The indices of refraction increase with higher iron content.<br />
<br />
Probably due to thermal shock (and/or heat treatment), some stones may show 4 (or even 8) different values per reading. This effect is named the "[[Refractometer#Kerez_effect|Kerez effect]]". Careful recutting of the stone will reveal that it is an outer-edge phenomenom [Dietrich, 1985].<br />
<br />
===Polariscope===<br />
<br />
Some dark colored tourmalines have a so called "closed axis" due to strong selective absorption in the direction of the optic axis and an interference figure may be hard (if not impossible) to find in that case.<br /><br />
Lighter colored stones may be cut with the optic axis perpendicular to the table and good interference figures can be found there.<br />
<br />
Some tourmalines show pseudo-biaxial (due to internal stress) interference figures on lateral rotation with a 2V up to 25° [Nesse, 2004; Dietrich, 1985].<br />
<br />
===Magnification===<br />
<br />
Tourmalines can be of type I to type III clarity grades.<br /><br />
Typical inclusions are:<br />
* Trichites (small thread-like twists)<br />
* Flattened liquid channels running parallel to the optic axis.<br />
* Liquid Veils<br />
* 2 and 3-Phase Inclusions<br />
* Hollow tubes<br />
<br />
[[Image:Tourmaline_veils_tubes.jpg|thumb|left|240px|Tourmaline with liquid veils, hollow tubes and phase inclusions<br />40X Magnification<br /> by Barbra Voltaire]]<br />
[[Image:Tourmaline_xls.jpg|thumb|left|240px|Tourmaline filled with negative crystals, oriented in random directions<br />80X Magnification<br /> by Barbra Voltaire]]<br />
<br clear="left" /><br />
<br />
==Phenomena==<br />
<br />
* Cat's-eyes<br />
<br />
==Treatments==<br />
<br />
Tourmalines may be heat treated to around 700° C to lighten the color, this is a stable alteration.<br /><br />
<br />
A process which seems to work well for deep saturated reds from Nigeria involves slowly ramping the furnace at a rate of 125° C per hour to 520° C, holding the latter temperature for 2 hours and then letting the furnace cool completely.<br /><br />
A deep red Nigerian stone in the gallery below has been heat treated several times as an experiment by Roger Dery, with the ramping done last.<br />
<br />
<gallery><br />
Image:RedTourmalineOval5 32ct.jpg|Nigerian red tourmaline. Heat treated at 360° C for 2 hours <br />
Image:RedTourmalineOval5 32v02.jpg|Nigerian red tourmaline. Heat treated at 410° C for 2 hours (same stone)<br />
Image:PinkTourmalineOval5 32ct.jpg|ramped up to 520° C over a three hour period. Held at 520° C for two hours, followed by cooling (same stone)<br />
</gallery><br />
<br />
<br />
Other treatments are irradiation - for example with cobalt-60 - (stable) and waxing of surface imperfections. The latter treatment is not stable.<br /><br />
Cobalt-60 irradiation gives rise pink and hot pink in tourmaline.<br />
<br />
==Sources==<br />
<br />
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)<br />
* ''The Tourmaline Group'' (1985) - Richard Dietrich ISBN 0442218575<br />
* ''Introduction to Optical mineralogy'' (2004) - William D. Nesse ISBN 0195149106<br />
* ''Refraction Anomalies in Tourmalines'' - R. Keith Mitchell, Journal of Gemmology Vol. 10, 194 (1967)</div>Doos