Difference between revisions of "Opal"

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[[Image:opal_tones.gif]]
 
[[Image:opal_tones.gif]]
 
===UV Reactions===
 
===UV Reactions===
Black opal is usually inert, but light varieties of opal, both common and precious may fluoresce in both LW and SW ultraviolet light.  Most natural opals phosphoresce green after exposure to LW ultraviolet light.  Synthetic opals do not.
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Black opal is usually inert, but light varieties of opal, both common and precious may fluoresce in both LW and SW ultraviolet light.  Some natural opals phosphoresce green after exposure to LW ultraviolet light.  Synthetic opals do not.
 
===Chelsea Colour Filter===
 
===Chelsea Colour Filter===
 
No diagnostic reaction
 
No diagnostic reaction

Revision as of 16:42, 9 January 2007

Exlamation mark.jpg
This section is currently under construction, do not edit until this message is gone
--Barbra 13:09, 9 January 2007 (PST)
Opal
Chemical composition SiO2+.H2O
Crystal system Amorphous
Hardness 5- 6 1/2
Refractive index 1.45 (+.020;-0.080)
Specific gravity 2.15 (+0.07;-0.90)
Lustre Vitreous
Phenomenom Play of Color
Precious Mexican Crystal Opal
Photo courtesy of Rick Martin, Art Cut Gems

Yellow Common Opal
Photo courtesy of Rick Martin, Art Cut Gems

Mintabe Opal Rub
Photo courtesy of Kevin Schwebel, Handmade Enterprises


Diagnostics

Description

Opal is a mineral species with an amorphous structure. It's composition is silicon dioxide with a variable amount of water. The amount of water (H2O) is usually between 2-10% by weight, although contents as high as 20% have been recorded.

In 2000, The Australian Gemstone Industry Council established nomenclature and classification standards for all types and origins of opals, This classification has been adopted internationally.
Opals can be broken down into 2 basic categories:

  • Precious Opal : any opal displaying play-of-color. This phenomenon is caused by the diffraction of white light thru a microscopic, orderly arrangement of silica spheres. This category includes white, black and bouldar opal.
  • Common Opal or Potch : These are varieties of opal that do not show a play-of-color. Although they share the same chemical compostion as precious opal, the silica shpheres they contain are randomly arranged.

Color

Opal tones.gif

UV Reactions

Black opal is usually inert, but light varieties of opal, both common and precious may fluoresce in both LW and SW ultraviolet light. Some natural opals phosphoresce green after exposure to LW ultraviolet light. Synthetic opals do not.

Chelsea Colour Filter

No diagnostic reaction

Treatments

  • Impregnation with oil, wax, or plastic.
  • Treatment with aniline dye, silver nitrate or sugar carbonized with acid.
  • Smoke impregnation.
  • Impregnation with black plastic.
  • Backing with foil, black paint or laquer.


Phenomena

Play of Color

Synthetics

Imitations

Slocum Stone: Glass Imitation (separation: magnification, RI, SG)
Japanese Plastic with Play of Color (separation: SG, Hardness: pressure with a pin will make a slight indentation)

Sources

  • Gems Their Sources, Descriptions and Identification 4th Edition (1990) - Robert Webster/ B.W. Anderson
  • Secrets of the Gem Trade (2003) - Richard W. Wise
  • A Student's Guide to Spectroscopy (2003) - Colin H. Winter
  • Gem Identification Made Easy 3rd edition (2006) - A.C. Bonanno/ Antoinette Matlins
  • GIA Gem Reference Guidefor the GIA Colored Stone & Gem Identification Courses

External Links