Difference between revisions of "Opal"

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'''Opals can be broken down into 2 basic categories:'''<br>
 
'''Opals can be broken down into 2 basic categories:'''<br>
  
* ''' ''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.<br>
+
* ''' ''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 boulder opal.  Precious opal can be further distinguished by types:<br>
 +
''Type 1:'' A single, solid piece of precious opal, having a uniform appearance and composition.  This is the type of opal most commonly used for jewelry<br>
 +
''Type 2:''Precious opal that is attached to its host rock (a non-opal) in the form of a layer or seam. Boulder opal is an example of this.  The opal is attached to a brown, iron-stained sandstone. <br>
 +
''Type 3:'' Matrix opal occurs when precious opal fills cracks and openings in the host rock.  The opal forms in pre-existing clay or sandstone.  This material is frequently dyed.
 +
''Doublets & Triplets:'' These assembled stones are not considered natural opals, although they do contain a layer of natural opal.
 +
 
 +
* ''' ''Common Opal or Potch'' ''': These are varieties of opal that do not show a play-of-color.  Although they share the same chemical composition as precious opal, the silica spheres they contain are randomly arranged. <br>
  
* ''' ''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. <br>
 
 
===Color===
 
===Color===
 
[[Image:opal_tones.gif]]
 
[[Image:opal_tones.gif]]

Revision as of 08:37, 24 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 boulder opal. Precious opal can be further distinguished by types:

Type 1: A single, solid piece of precious opal, having a uniform appearance and composition. This is the type of opal most commonly used for jewelry
Type 2:Precious opal that is attached to its host rock (a non-opal) in the form of a layer or seam. Boulder opal is an example of this. The opal is attached to a brown, iron-stained sandstone.
Type 3: Matrix opal occurs when precious opal fills cracks and openings in the host rock. The opal forms in pre-existing clay or sandstone. This material is frequently dyed. Doublets & Triplets: These assembled stones are not considered natural opals, although they do contain a layer of natural opal.

  • Common Opal or Potch : These are varieties of opal that do not show a play-of-color. Although they share the same chemical composition as precious opal, the silica spheres they contain are randomly arranged.

Color

Opal tones.gif
Judge the body color of an opal "face up".
Example:
If an opal has very dark potch on the back, giving it the appearance of N5, it should be graded as N5.

Transparency

Opals can vary in degrees of transparency from transparent to opaque. When an opal is transparent or semi-transparent it is referred to "crystal". This is true regardless of the body tone. "Crystal" refers to the glass-like appearance of the gem, NOT a crystalline structure.

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