OPAL ENHANCEMENT AND TREATMENT

Opal is subject to a number of treatments most of which are traditional and well known to the jeweler/gemologist. The two best known are smoking and sugaring. Each aim to darken the body color in order to enhance the play of color.

Recently several opal dealers have begun experimenting with plastic polymers in an attempt to enhance the clarity and/or stabilize the gem to prevent crazing and cracking. Dr. David Lipman a professional chemist and part time opalholic has begun treating opal with CR-39, a polymer used in the manufacture of lenses for diving masks. Developed by PPG Industries, CR-39 has a refractive index of 1.43-1.46 which overlaps opal's normal RI range of 1.44-1.46. Lipman reports good success in stabilizing rough from problem areas such as Virgin Valley, Utah and the Mintabe field in Australia.

Lipman's process involves placing the opal to be treated in a vacuum chamber, a method similar to the opticon process used to treat emerald. The vacuum accomplishes two objectives first it allows the polymer to be drawn into cracks and fissures that break the surface of the material and second it creates the airless environment required for the polymer to set-up and harden. This later (hardening) step is rarely used in emerald treatment.

Dr. Lipman was kind enough to provide me with nine samples of opal treated with his process. I examined these samples using standard instruments available to the average jeweler/gemologist with the following results:

In samples where the rough had been stabilized before cutting I found evidence of jagged, scar-like polishing lines, resembling draglines, probably the result of undercutting where the softer polymer was eaten away by the action of the polishing wheel. These lines where quite evident using a standard jewelers 10x loupe positioned so that overhead light reflected off the surface of the stone.

I next examined several samples where Dr. Lipman attempted to mend cracks in cut gems. To my surprise, despite the overlap of refractive index, cracks treated with CR-39 were visible to the naked eye when viewed in normal lighting. Under low magnification (10-20x), all of the treated stones showed a white, opaque flaky fingerprint inside the fracture. This appears to be the result of the polymer lifting away from the surface of the fracture. This may be the result of differing rates of thermal expansion between the polymer and the host material. The fracture line was also visible on the surface of the stone viewed in reflected light. Filled areas showed no reaction to ultraviolet light and refractive index and specific gravity were unaffected.

CONCLUSIONS AND CAVEAT

Based on the samples provided, it would appear that polymer treatment with CR-39 while useful in stabilizing softer varieties of opal rough to permit cutting is
easily detectable when used to enhance the clarity of cut opal. However, as Dr. Lipman points, out there are a number of polymers currently on the market that can be used to treat opal. "It is just a matter of time before the perfect match is found."

SPEAKING OF PLASTIC

Ed Hilton of Opal America mentioned a new plastic opal simulant called Opalite that he would have difficulty to detecting if mounted. Mr. Yung S. Kim of Universal

Canal, Inc., the U. S. distributor, sent me a sample cabochon.
Visually the sample strongly resembles a white based (Coober Pedy) opal of medium intensity with predominantly green play of color. The specific gravity of opalite is 1.18-1.20 which makes it ultra light even for plastic. When viewed from the back opalite has an unnatural looking sheen similar to abalone shell. These two factors make identification of unmounted opalite fairly easy.

Bezel mounted and backed, identification of opalite can be quite another matter. According to literature supplied by the distributor, opalite has "virtually the same structure as genuine opals" with sub-microscopic spheres of synthetic polymer used as a replacement for the silica found in natural opal. This would account for the fact that the play of color, even under magnification, in both reflected and transmitted light, is quite convincing. There is no sign of the lizard skin effect found in true synthetics. Thus, visual identification is not to be relied upon to separate opalite from natural opal in mounted stones.

I next subjected the sample to standard gemological testing. Subjected to ultraviolet light opalite exhibited a chalky white florescence which was stronger long wave. These reactions parallel some light colored natural opal. A spot refractive index reading of 1.48 is too high for opal, which should be no higher than 1.46 and is normally lower. When rotated in the polariscope the sample showed a distinctive anomalous double refraction (ADR) which took the form of a strong reddish glow, which alternated with the normal body color as, the stone was rotated 360 degrees between crossed polaroids. According to Mr. Yung Opalite was first introduced to the American market at the February 1993 Tucson show but, has been on the market in Asia for about ten years. Careful testing with the polariscope and refractometer should be sufficient to separate opalite from opal. Given the difficulty of obtaining accurate spot refractive index readings I recommend that both tests be used in conjunction. Positive identification proves difficult whan mountings preclude the use of the polariscope and refractometer. "