Difference between revisions of "Opal"

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(Imitations)
 
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{{edit|--[[User:Barbra|Barbra]] 13:09, 9 January 2007 (PST)}}
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{{opal}}
  
{{opal}}
 
  
  
[[Image:YelOpal.jpg|left|framed|Yellow Common Opal<br />Photo courtesy of Rick Martin, Art Cut Gems]]<br clear="left" />
 
[[Image:Whiteopal.JPG|left|framed|Mintabe Opal Rub<br /> Photo courtesy of Kevin Schwebel, Handmade Enterprises]]<br clear="left" /><br>
 
  
 
==Diagnostics==
 
==Diagnostics==
  
 
===Description===
 
===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 (H<sub>2</sub>O) is usually between 2-10% by weight, although contents as high as 20% have been recorded. <br>
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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 (H<sub>2</sub>O) is usually between 2-10% by weight, although contents as high as 20% have been recorded. When opals undergo dehydration as a result of exposure and time they have a tendency to become crazed, showing minute cracks on the polished surfaces.<br>
 
<br>
 
<br>
 
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.  <br>
 
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.  <br>
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* ''' ''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>
 
* ''' ''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>
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[[image:Whiteopal.JPG|left|thumb|225px|Mintabe Opal Rub<br /> Photo courtesy of Kevin Schwebel, Handmade Enterprises]]<br clear="left" />
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''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 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 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>
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* ''' ''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 composition as precious opal, the silica spheres they contain are randomly arranged. <br>
  
[[image:YelOpal.jpg|left|thumb|225px|Yellow Common Opal<br />Photo courtesy of Rick Martin, Art Cut Gems]]
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[[image:YelOpal.jpg|left|thumb|225px|Yellow Common Opal<br />Photo courtesy of Rick Martin, Art Cut Gems]]<br clear="left" />
  
 
===Color===
 
===Color===
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[[image:opalmex.jpg|left|thumb|225px|Precious Mexican Crystal Opal
 
[[image:opalmex.jpg|left|thumb|225px|Precious Mexican Crystal Opal
Photo courtesy of Rick Martin, Art Cut Gems]]<br><br><br><br><br><br><br><br><br><br><br><br>
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Photo courtesy of Rick Martin, Art Cut Gems]]<br clear="left" />
 
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.
 
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.
  
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==Treatments==
 
==Treatments==
 
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[[image:Carbonized_opal.jpg|left|frame|Opal treated with carbonized sugar and acid
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]]<br clear="left" />
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* Treatment with aniline dye, silver nitrate, or sugar carbonized with acid.<br>
 
* Impregnation with oil, wax, or plastic. <br>
 
* Impregnation with oil, wax, or plastic. <br>
* Treatment with aniline dye, silver nitrate or sugar carbonized with acid.<br>
 
 
* Smoke impregnation. <br>
 
* Smoke impregnation. <br>
 
* Impregnation with black plastic.<br>
 
* Impregnation with black plastic.<br>
* Backing with foil, black paint or laquer.
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* Backing with foil, black paint, or laquer.
 
 
  
 
==Phenomena==
 
==Phenomena==
Play of Color
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Play-of-Color
 
==Synthetics==
 
==Synthetics==
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The firms of Gilson in France and Chatham in the United States currently create synthetic opals. They are produced with white and black body colors. Chatham also markets a "crystal" type which exhibits a colorless body with play of color.<br>
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This laboratory created material was first marketed by Pierre Gilson, Sr. in 1974. It involves a 3 stage process involving the purification of chemicals involving fractional distillation. This is characteristically an elaborate process invoving consolidation of these refined chemicals under water pressure. The result is the creation of microscopic cristobolite silica spheres (SiO2) of uniform size. They are allowed to settle in the containment vessel for over a year in a hydrous solution of controlled pressure and acidity. Then, a hydrostatic press is utilized to consolidate the microspheres from a liquid phase into a solid phase, resulting in synthetic opal rough which can be cut into cabochons.<br>
  
 
==Imitations==
 
==Imitations==
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Slocum Stone: Glass Imitation (separation: magnification, RI, SG)<br>
 
Slocum Stone: Glass Imitation (separation: magnification, RI, SG)<br>
 
Japanese Plastic with Play of Color (separation: SG, Hardness: pressure with a pin will make a slight indentation)
 
Japanese Plastic with Play of Color (separation: SG, Hardness: pressure with a pin will make a slight indentation)
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== G&G Articles on Opal 1934-1980==
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The GIA has published all the [http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/index-back-issues-1934-80.html G&G's from 1934 until 1980 online]. The organization of the list by subject was done by [http://archive.org/details/GillsHistoricalIndexToGemsAndJewelryOnline-ByJosephO.Gill2009in Joseph Gill].
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<ul>
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<li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1937.pdf Winter 1937, <b>The Australian Black Opal</b>, p. 137, 2pp.]
 +
</li>
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<li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1938.pdf Winter 1938, <b>Opal (with glossary of varieties)</b>, by Briggs, p. 201, 2pp.]</li>
 +
<li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1946.pdf Winter 1946, <b>New Australian Opal Fields</b>, p. 364, 3pp.]</li>
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<li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1947.pdf Fall 1947, <b>Mexican Treated Black Opal</b>, p. 475, 2pp.]</li>
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<li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1950.pdf Fall 1950, <b>Opal Industry of Australia, past and present</b>, p. 334, 8pp.]
 +
</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1950.pdf Winter 1950, <b>British Collector Now Owns the Pandora Opal</b>, p. 381, 1p.]
 +
</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/spring-1953.pdf Spring 1953, <b>Mexican Opal</b>, by Foshag, p. 278, 5pp.]
 +
</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1959.pdf Fall 1959, <b>Opal of Australia</b>, p. 323, 11pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1965.pdf Winter 1965, <b>The Black Opals of Lightning Ridge</b>, p. 355, 4pp.]
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</li>
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<li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1971.pdf Fall 1971, <b>The Story of the White Cliffs Opal Field, Australia</b>, p. 334, 10pp]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1971.pdf Fall 1971, <b>A carved 288.25 ct</b>. Mexican opal, p. 357, 2pp.]
 +
</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1972.pdf Fall 1972, <b>Opal with play-of-colors found in Arizona</b>, p. 91, 1p.]
 +
</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1976.pdf Fall 1976, <b>Opalescent Sandstone from Louisiana</b>, p. 201, 1p.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1979.pdf Fall 1979,<b> COOBER PEDY - The Opal Town</b>, by J. Stone, p. 213, 4pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/spring-1980.pdf Spring 1980, <b>GEMLURE, Opal: Smolder of Fortune?,</b> by Cheri Lesh, p. 283, 6pp.]
 +
</li>
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<li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/Nov-Dec-1935.pdf Nov.-Dec. 1935, <b>New type of opal doublet</b>, p. 344, 1p.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1948.pdf Fall 1948, <b>Variation in opal properties per locality</b>, p. 90, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1953.pdf Winter 1953, <b>Cause of Color in Opal</b>, by Leechman, p. 361, 4pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/spring-1954.pdf Spring 1954, <b>The Structure and Optical Behavior of Iridescent Opal</b>, p. 21, 6pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/spring-1955.pdf Spring 1955, <b>Lattice Structure in Precious Opal</b>, by Leechman, p. 154, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1959.pdf Fall 1959, <b>Treated black opal</b>, p. 343, 1p.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1962.pdf Fall 1962, <b>Treated black opal</b>, p. 336, 3pp. (See also Winter 1962, p. 380, 1p.)]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/spring-1964.pdf Spring 1964, <b>Treated Black Opal</b>, by Gübelin, p. 157, 3pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1964.pdf Fall 1964, <b>Opal triplet; black onyx backed, quartz topped</b>, p. 221, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1964.pdf Winter 1964, <b>Treated black opal and its patchy iridescence</b>, p. 242, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/summer-1965.pdf Summer 1965, <b>The Origin of Color in Opal</b>, p. 291, 8pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/summer-1966.pdf Summer 1966, <b>A cat's-eye opal</b>, p. 60, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/spring-1967.pdf Spring 1967, <b>Oolitic opal</b>, p. 149, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/summer-1967.pdf Summer 1967, <b>An opal that loses its color when water soaked</b>, p. 179, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1967.pdf Fall 1967, <b>Formation of Precious Opal</b>, p. 194, 5pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1967.pdf Fall 1967, <b>Oolitic opal</b>, p. 199, 1p.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1967.pdf Fall 1967, <b>Opal substitutes (crushed opal in resin)</b>, p. 219, 3pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/spring-1968.pdf Spring 1968, <b>Opal and rock crystal doublets</b>, p. 282, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall_1968.pdf Fall 1968, <b>Rare cat's-eye opals</b>, p. 342, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1968.pdf Winter 1968, <b>Odd leaf-like inclusion in an opal</b>, p. 380, 2pp]
 +
</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/spring-1969.pdf Spring 1969, <b>Opal imitation</b>, p. 21, 1p.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/spring-1969.pdf Spring 1969, <b>Some unusual opal inclusions</b>, p. 26, 3pp]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/summer-1969.pdf Summer 1969, <b>Chrysocolla opal (first seen)</b>, p. 67, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1969.pdf Fall 1969, <b>Smoke-treated opal</b>, p. 91, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1969.pdf Fall 1969, <b>Some unusual opals</b>, p. 97, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1969.pdf Winter 1969, <b>Artificially colored opals</b>, p. 117, 1p.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1969.pdf Winter 1969, <b>An unusual sugar-and-acid-treated opal</b>, p. 127, 1p.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/spring-1970.pdf Spring 1970, <b>A porous white opal impregnated with black plastic</b>, p. 148, 1p.]
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</li><li>
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[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1970.pdf Winter 1970, <b>Stained black opal</b>, p. 249, 3pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/spring-1971.pdf Spring 1971, <b>Nomenclature of black opal</b>, p. 284, 2pp]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/summer-1971.pdf Summer 1971,<b> A new opal, treated black, with very low properties</b>, p. 317, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/summer-1971.pdf Summer 1971, <b>A very unusual opal</b>, p. 321, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1971.pdf Fall 1971,<b> Black-dyed oolitic opal</b>, p. 351, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1971.pdf Winter 1971, <b>Natural black oolitic opal</b>, p. 382, 1p.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/spring-1972.pdf Spring 1972, <b>New synthetic opal from Gilson</b>, p. 18, 1p.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/spring-1972.pdf Spring 1972, <b>Sugar-treated black opal</b>, p. 21, 1p.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/spring-1972.pdf Spring 1972, <b>Easily testing a mounted opal doublet</b>, p. 24, 3pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/summer-1972.pdf Summer 1972, <b>An opal cabochon in which the chipped back was epoxy filled</b>, p. 43, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1972.pdf Fall 1972, <b>High transparency in some black opals</b>, p. 88, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1972.pdf Winter 1972, <b>Sugar-treated black opal, demonstration</b>, p. 117, 1p.]
 +
</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/summer-1973.pdf Summer 1973, <b>Gilson synthetic opal</b>, p. 175, 3pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1973.pdf Winter 1973, <b>A new opal doublet to look like boulder opal</b>, p. 233, 1p.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/summer-1974.pdf Summer 1974, <b>Treating Matrix Opal</b>, p. 306, 3pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/summer-1974.pdf Summer 1974, <b>Synthetic opal</b>, p. 309, 3pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1974.pdf Fall 1974, <b>Imitation opal on matrix</b>, p. 331, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1974.pdf Fall 1974, <b>Gilson synthetic opal (black)</b>, p. 343, 1p.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1974.pdf Winter 1974, <b>An excellent opal imitation</b>, p. 362, 3pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1974.pdf Winter 1974, <b>A black opal doublet, with uneven joint</b>, p. 364, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/spring-1975.pdf Spring 1975, <b>A strange, dried-up opal</b>, p. 26, 1p.
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/summer-1976.pdf Summer 1976, <b>Crystal inclusions in Mexican opal</b>, p. 174, 1p.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/summer-1976.pdf Summer 1976, <b>The Slocum imitation opal put on the market</b>, p. 185, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1976.pdf Fall 1976, <b>Non-Single-Crystal Synthetic (synthetic Gilson opal)</b>, by Nassau, p. 194, 5pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1976.pdf Fall 1976, <b>A ball-like inclusion and curved striae in a botryoidal opal</b>, p. 213, 2pp.]
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</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1976.pdf Winter 1976, <b>Opal doublet to imitate boulder opal</b>, p. 115, 2pp.]
 +
</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1976.pdf Winter 1976, <b>Gilson synthetic opal</b>, p. 126, 1p.]
 +
</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1977.pdf Winter 1977, <b>Gilson black opal triplet</b>, p. 236, 1p.]
 +
</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/winter-1977.pdf Winter 1977, <b>Observations on the Slocum Stone</b>, p. 252, 5pp. (See also Summer 1977, p. 301, 1p. and p. 308, 1p.)]
 +
</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/summer-1978.pdf Summer 1978, <b>Plastic Impregnated Gem Opal</b>, by D. Vincent Manson, p. 49, 9p.]
 +
</li><li>[http://www.gia.edu/research-resources/gems-gemology/back-issue-archive/fall-1979.pdf Fall 1979, <b>Plastic Impregnated Opal and Plastic Opal</b>, p. 219, 1p.]
 +
</li></ul>
  
 
==Sources==
 
==Sources==
  
 
* ''Gems Their Sources, Descriptions and Identification'' 4th Edition (1990) - Robert Webster/ B.W. Anderson
 
* ''Gems Their Sources, Descriptions and Identification'' 4th Edition (1990) - Robert Webster/ B.W. Anderson
* ''Secrets of the Gem Trade'' (2003) - Richard W. Wise
+
* [http://www.amazon.com/gp/product/0972822380?ie=UTF8&tag=gemsandwhywelove&link_code=as3&camp=211189&creative=373489&creativeASIN=0972822380 Secrets of the Gem Trade, by Richard W. Wise]
 
* ''A Student's Guide to Spectroscopy'' (2003) - Colin H. Winter
 
* ''A Student's Guide to Spectroscopy'' (2003) - Colin H. Winter
 
* ''Gem Identification Made Easy'' 3rd edition (2006) - A.C. Bonanno/ Antoinette Matlins
 
* ''Gem Identification Made Easy'' 3rd edition (2006) - A.C. Bonanno/ Antoinette Matlins
* ''GIA Gem Reference Guide''for the GIA Colored Stone & Gem Identification Courses
+
* ''GIA Gem Reference Guide'' for the GIA Colored Stone & Gem Identification Courses
  
 
==External Links==
 
==External Links==
  
 
* [http://www.opalsdownunder.com.au/articles/nomenclature.php Opal Nomenclature and Classification]
 
* [http://www.opalsdownunder.com.au/articles/nomenclature.php Opal Nomenclature and Classification]

Latest revision as of 20:05, 21 August 2012

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



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. When opals undergo dehydration as a result of exposure and time they have a tendency to become crazed, showing minute cracks on the polished surfaces.

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:
Mintabe Opal Rub
Photo courtesy of Kevin Schwebel, Handmade Enterprises

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

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

Precious Mexican Crystal Opal Photo courtesy of Rick Martin, Art Cut Gems

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

Opal treated with carbonized sugar and acid

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

Phenomena

Play-of-Color

Synthetics

The firms of Gilson in France and Chatham in the United States currently create synthetic opals. They are produced with white and black body colors. Chatham also markets a "crystal" type which exhibits a colorless body with play of color.
This laboratory created material was first marketed by Pierre Gilson, Sr. in 1974. It involves a 3 stage process involving the purification of chemicals involving fractional distillation. This is characteristically an elaborate process invoving consolidation of these refined chemicals under water pressure. The result is the creation of microscopic cristobolite silica spheres (SiO2) of uniform size. They are allowed to settle in the containment vessel for over a year in a hydrous solution of controlled pressure and acidity. Then, a hydrostatic press is utilized to consolidate the microspheres from a liquid phase into a solid phase, resulting in synthetic opal rough which can be cut into cabochons.

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)

G&G Articles on Opal 1934-1980

The GIA has published all the G&G's from 1934 until 1980 online. The organization of the list by subject was done by Joseph Gill.


Sources

  • Gems Their Sources, Descriptions and Identification 4th Edition (1990) - Robert Webster/ B.W. Anderson
  • Secrets of the Gem Trade, by 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 Guide for the GIA Colored Stone & Gem Identification Courses

External Links