Difference between revisions of "Diaspore"

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Diaspore as a plausible gemstone has been reported first around 1977 when a large enough crystal was examined at the Gem Testing Laboratory in London. This, usually colorless, material showed good color-change from pale bluish-green to pale pinkish-brown. Turkey was the origin of this color-change material.
 
Diaspore as a plausible gemstone has been reported first around 1977 when a large enough crystal was examined at the Gem Testing Laboratory in London. This, usually colorless, material showed good color-change from pale bluish-green to pale pinkish-brown. Turkey was the origin of this color-change material.
 
When Zultanite Gems LLC aquired the mining location, this color-change variety of diaspore was marketed under the tradename "zultanite®". At present (2007) sole distribution rights of "zultanite®" are granted to GemsTV.
 
When Zultanite Gems LLC aquired the mining location, this color-change variety of diaspore was marketed under the tradename "zultanite®". At present (2007) sole distribution rights of "zultanite®" are granted to GemsTV.
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==Chemical composition==
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AlOOH + Mn (or Fe) as impurity (a dimorph of boehmite).<br />
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Mn or Fe may substitute Al in the lattice.
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 +
==Crystallography==
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Orthorhombic.
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Habit: crystals are elongated plates (tabular); acicular needles also massive; foilated.
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 +
==Diagnostics==
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 +
Diaspore may be confused with [[peridot]] due to the strong doubling of back facets, but diaspore is usually much paler in color. Iron-rich green sapphire has an absorption sectrum close to sapphire.
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 +
===Color===
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Colorless, pale bluish-green to pale green, pale pinkish-brown to pale brown.
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===Diaphaneity===
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Transparent to opaque.
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===Specific Gravity===
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S.G.:3.2 - 3.5 (mean = 3.4)
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===Cleavage===
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Perfect in one direction {010} and imperfect on {110} and {210}.
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===Refractometer===
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Optic nature: Biaxial positive.<br />
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Refractive index range: n<sub>&alpha;</sub> = 1.682 - 1.702, n<sub>&Beta;</sub> = 1.705 - 1.725 and n<sub>&gamma;</sub> = 1.730 - 1.752. Mean = 1.702 - 1.750.<br />
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Maximum birefringence: 0.040 - 0.048.
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===Spectrum===
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Not diagnostic but Turkish stones 471, 463, 454 (much similar to iron-rich sapphire).
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===Luminescence===
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UV fluorescence: May show dull pale yellow (LW) and green (SW)
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===Pleochroism===
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Strong depending on body colour and source. Blue / Pale green and rose to dark red.
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==Simulants==
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{| {{table}} width="70%" style="margin-left:0"
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|-
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| [[Sapphire]] || No strong doubling of facets; higher heft; higher R.I., unixial figure.
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|-
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| [[Sillimanite]] || Lower S.G., lower RI and lower birefringence.
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|-
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| [[Peridot]] || Lower RI and slightly lower birefringence.
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|}
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==Occurrence==
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Geological occurrence:<br />
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In metamorphosed limestones, chloritic schists and altered igneous rocks. Also in bauxite deposits.
 +
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Geographical locations:<br />
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Memaris, Turkey (gemmy pale brown); Chester, Massachusetts (with corundum); Pennysylvania (fine transparent colourless to brown); Hungary; South Africa in Postmasburg district - Manganiferous variety; Cornwall, England; Greenland; Norway; Sweden; France; Switzerland; Germany; Greece; USSR; Japan; China; Turkey.
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 +
==References==
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* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)
 +
* ''Colour Encyclopedia'' - J.E. Arem ISBN 0412989115
 +
* ''Textbook of mineralogy'' - E.S. Dana
 +
* ''Introduction to Optical Mineralogy'' (2004) - William D. Nesse ISBN 0195149106

Revision as of 10:54, 30 August 2007

Diaspore
Chemical composition AlO(OH)
Crystal system Orthorhombic
Habit Tabular, sometimes fibrous
Cleavage Perfect to imperfect
Fracture Conchoidal
Luster Bright vitreous to pearly (on cleavage surface).
Hardness 6.5 - 7
Optic nature Biaxial +
Refractive index 1.682 - 1.752
Birefringence 0.040 - 0.048
Specific gravity 3.2 - 3.5
Lustre Vitreous
Pleochroism Moderate
Color-changing diaspore from green in daylight to brown in incandescent light - also known as “Zultanite” in the gem market.
Photo courtesy of Chaman Golecha, Gem Testing Laboratory, Jaipur.


Diaspore as a plausible gemstone has been reported first around 1977 when a large enough crystal was examined at the Gem Testing Laboratory in London. This, usually colorless, material showed good color-change from pale bluish-green to pale pinkish-brown. Turkey was the origin of this color-change material. When Zultanite Gems LLC aquired the mining location, this color-change variety of diaspore was marketed under the tradename "zultanite®". At present (2007) sole distribution rights of "zultanite®" are granted to GemsTV.

Chemical composition

AlOOH + Mn (or Fe) as impurity (a dimorph of boehmite).
Mn or Fe may substitute Al in the lattice.

Crystallography

Orthorhombic. Habit: crystals are elongated plates (tabular); acicular needles also massive; foilated.

Diagnostics

Diaspore may be confused with peridot due to the strong doubling of back facets, but diaspore is usually much paler in color. Iron-rich green sapphire has an absorption sectrum close to sapphire.

Color

Colorless, pale bluish-green to pale green, pale pinkish-brown to pale brown.

Diaphaneity

Transparent to opaque.

Specific Gravity

S.G.:3.2 - 3.5 (mean = 3.4)

Cleavage

Perfect in one direction {010} and imperfect on {110} and {210}.

Refractometer

Optic nature: Biaxial positive.
Refractive index range: nα = 1.682 - 1.702, nΒ = 1.705 - 1.725 and nγ = 1.730 - 1.752. Mean = 1.702 - 1.750.
Maximum birefringence: 0.040 - 0.048.

Spectrum

Not diagnostic but Turkish stones 471, 463, 454 (much similar to iron-rich sapphire).

Luminescence

UV fluorescence: May show dull pale yellow (LW) and green (SW)

Pleochroism

Strong depending on body colour and source. Blue / Pale green and rose to dark red.

Simulants

Sapphire No strong doubling of facets; higher heft; higher R.I., unixial figure.
Sillimanite Lower S.G., lower RI and lower birefringence.
Peridot Lower RI and slightly lower birefringence.

Occurrence

Geological occurrence:
In metamorphosed limestones, chloritic schists and altered igneous rocks. Also in bauxite deposits.

Geographical locations:
Memaris, Turkey (gemmy pale brown); Chester, Massachusetts (with corundum); Pennysylvania (fine transparent colourless to brown); Hungary; South Africa in Postmasburg district - Manganiferous variety; Cornwall, England; Greenland; Norway; Sweden; France; Switzerland; Germany; Greece; USSR; Japan; China; Turkey.

References

  • Gems, Their Sources, Descriptions and Identification 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)
  • Colour Encyclopedia - J.E. Arem ISBN 0412989115
  • Textbook of mineralogy - E.S. Dana
  • Introduction to Optical Mineralogy (2004) - William D. Nesse ISBN 0195149106