A parcel of material represented as boulder opal was purchased at the February 2000 Tucson show by Parisian lapidary Alexandre Wolkonsky. The free-form, flat cabochons ranged from 1.26 to 10.67 ct. Because of the unusual stated locality (China), and the low price of the parcel compared to equivalent pieces from Australia, the gemological properties of these samples were carefully studied. Under magnification, it was clear that the material was not natural boulder opal, but rather an assembled quadruplet with the following four layers (from top to bottom):
• Natural opal top, with a typical appearance and refractive index (1.44)
• A very thin, black layer of apparently even thickness
• A brown layer of homogeneous color but irregular thickness
• An ironstone base typical of that seen on boulder opal, with limonite,
clay, and an occasional opal veinlet
The homogeneous brown and black layers did not melt on contact with a hot point, but rather they softened and flaked off. In some of the samples examined, the brown layers contained gas bubbles, which identified the material as a plastic. This was confirmed with a Bruker RFS 100 FTRaman microspectrometer, which revealed a broad signal around 3000 cm-1 that is typical of C–H groups found in organic matter such as plastic (see B. Schrader, Raman/Infrared Atlas of Organic Compounds, VCH, Weinheim, Germany, 1989, pp. N-01–N-11). The black layer was probably added to enhance color contrast, as does the thin layer of black potch that is sometimes seen backing “black” boulder opal (see, e.g., R .W. Wise, “Queensland boulder opal,” Spring 1993 Gems & Gemology, pp. 4–15).
Most remarkably, the bottom surface of the natural opal layer and the top surface of the ironstone layer were not always flat and planar. In some stones, these layers were clearly curved (see figure; photomicrograph by E. Fristch, magnified 25×), in contrast to other assembled stones, which generally contain planar contact surfaces. The non-coincidence of these two surfaces was compensated by the variable thickness of the brown plastic layer.
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