16.50: Opal

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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. Its 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:

File:Whiteopal.JPG

Figure \(\PageIndex{1}\): 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.

File:YelOpal.jpg

Figure \(\PageIndex{2}\): Yellow Common Opal
Photo courtesy of Rick Martin, Art Cut Gems

Color

Figure \(\PageIndex{3}\)

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

File:Opalmex.jpg

Figure \(\PageIndex{4}\): 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 as "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

Figure \(\PageIndex{5}\): 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 involving the consolidation of these refined chemicals under water pressure. The result is the creation of microscopic cristobalite 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.

Winter 1937, The Australian Black Opal, p. 137, 2pp.
Winter 1938, Opal (with glossary of varieties), by Briggs, p. 201, 2pp.
Winter 1946, New Australian Opal Fields, p. 364, 3pp.
Fall 1947, Mexican Treated Black Opal, p. 475, 2pp.
Fall 1950, Opal Industry of Australia, past and present, p. 334, 8pp.
Winter 1950, British Collector Now Owns the Pandora Opal, p. 381, 1p.
Spring 1953, Mexican Opal, by Foshag, p. 278, 5pp.
Fall 1959, Opal of Australia, p. 323, 11pp.
Winter 1965, The Black Opals of Lightning Ridge, p. 355, 4pp.
Fall 1971, The Story of the White Cliffs Opal Field, Australia, p. 334, 10pp
Fall 1971, A carved 288.25 ct. Mexican opal, p. 357, 2pp.
Fall 1972, Opal with play-of-colors found in Arizona, p. 91, 1p.
Fall 1976, Opalescent Sandstone from Louisiana, p. 201, 1p.
Fall 1979, COOBER PEDY - The Opal Town, by J. Stone, p. 213, 4pp.
Spring 1980, GEMLURE, Opal: Smolder of Fortune?, by Cheri Lesh, p. 283, 6pp.
Nov.-Dec. 1935, New type of opal doublet, p. 344, 1p.
Fall 1948, Variation in opal properties per locality, p. 90, 2pp.
Winter 1953, Cause of Color in Opal, by Leechman, p. 361, 4pp.
Spring 1954, The Structure and Optical Behavior of Iridescent Opal, p. 21, 6pp.
Spring 1955, Lattice Structure in Precious Opal, by Leechman, p. 154, 2pp.
Fall 1959, Treated black opal, p. 343, 1p.
Fall 1962, Treated black opal, p. 336, 3pp. (See also Winter 1962, p. 380, 1p.)
Spring 1964, Treated Black Opal, by Gübelin, p. 157, 3pp.
Fall 1964, Opal triplet; black onyx backed, quartz topped, p. 221, 2pp.
Winter 1964, Treated black opal and its patchy iridescence, p. 242, 2pp.
Summer 1965, The Origin of Color in Opal, p. 291, 8pp.
Summer 1966, A cat's-eye opal, p. 60, 2pp.
Spring 1967, Oolitic opal, p. 149, 2pp.
Summer 1967, An opal that loses its color when water soaked, p. 179, 2pp.
Fall 1967, Formation of Precious Opal, p. 194, 5pp.
Fall 1967, Oolitic opal, p. 199, 1p.
Fall 1967, Opal substitutes (crushed opal in resin), p. 219, 3pp.
Spring 1968, Opal and rock crystal doublets, p. 282, 2pp.
Fall 1968, Rare cat's-eye opals, p. 342, 2pp.
Winter 1968, Odd leaf-like inclusion in an opal, p. 380, 2pp
Spring 1969, Opal imitation, p. 21, 1p.
Spring 1969, Some unusual opal inclusions, p. 26, 3pp
Summer 1969, Chrysocolla opal (first seen), p. 67, 2pp.
Fall 1969, Smoke-treated opal, p. 91, 2pp.
Fall 1969, Some unusual opals, p. 97, 2pp.
Winter 1969, Artificially colored opals, p. 117, 1p.
Winter 1969, An unusual sugar-and-acid-treated opal, p. 127, 1p.
Spring 1970, A porous white opal impregnated with black plastic, p. 148, 1p.
Winter 1970, Stained black opal, p. 249, 3pp.
Spring 1971, Nomenclature of black opal, p. 284, 2pp
Summer 1971, A new opal, treated black, with very low properties, p. 317, 2pp.
Summer 1971, A very unusual opal, p. 321, 2pp.
Fall 1971, Black-dyed oolitic opal, p. 351, 2pp.
Winter 1971, Natural black oolitic opal, p. 382, 1p.
Spring 1972, New synthetic opal from Gilson, p. 18, 1p.
Spring 1972, Sugar-treated black opal, p. 21, 1p.
Spring 1972, Easily testing a mounted opal doublet, p. 24, 3pp.
Summer 1972, An opal cabochon in which the chipped back was epoxy filled, p. 43, 2pp.
Fall 1972, High transparency in some black opals, p. 88, 2pp.
Winter 1972, Sugar-treated black opal, demonstration, p. 117, 1p.
Summer 1973, Gilson synthetic opal, p. 175, 3pp.
Winter 1973, A new opal doublet to look like boulder opal, p. 233, 1p.
Summer 1974, Treating Matrix Opal, p. 306, 3pp.
Summer 1974, Synthetic opal, p. 309, 3pp.
Fall 1974, Imitation opal on matrix, p. 331, 2pp.
Fall 1974, Gilson synthetic opal (black), p. 343, 1p.
Winter 1974, An excellent opal imitation, p. 362, 3pp.
Winter 1974, A black opal doublet, with uneven joint, p. 364, 2pp.
[www.gia.edu/research-resources/gems-gemology/back-issue-archive/spring-1975.pdf Spring 1975, A strange, dried-up opal, p. 26, 1p.
Summer 1976, Crystal inclusions in Mexican opal, p. 174, 1p.
Summer 1976, The Slocum imitation opal put on the market, p. 185, 2pp.
Fall 1976, Non-Single-Crystal Synthetic (synthetic Gilson opal), by Nassau, p. 194, 5pp.
Fall 1976, A ball-like inclusion and curved striae in a botryoidal opal, p. 213, 2pp.
Winter 1976, Opal doublet to imitate boulder opal, p. 115, 2pp.
Winter 1976, Gilson synthetic opal, p. 126, 1p.
Winter 1977, Gilson black opal triplet, p. 236, 1p.
Winter 1977, Observations on the Slocum Stone, p. 252, 5pp. (See also Summer 1977, p. 301, 1p. and p. 308, 1p.)
Summer 1978, Plastic Impregnated Gem Opal, by D. Vincent Manson, p. 49, 9p.
Fall 1979, Plastic Impregnated Opal and Plastic Opal, p. 219, 1p.

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

Opal Nomenclature and Classification