A pair of milky white polished opals, weighing 2.66 and 2.48 ct, were recently examined at the Carlsbad laboratory (figure 1). Spot RI readings of 1.44 and 1.43, respectively, were consistent with opal. The stoneswere placed under long-wave ultraviolet light (LWUV) as part of the routine testing.
Upon removal from the UV light source, it was apparent that the opals’ color had changed to a bright orangy red. Further gemological testing confirmed that the specimens had been impregnated with an unknown photochromic substance that changed color when exposed to UV light. The color change (figure 2) occurred after approximately one minute of exposure. The orangy red color faded completely about five minutes after removal from the UV source. Furthermore, when the specimens were placed near a window, sunlight provided enough UV radiation to fully induce the orangy red color. This reversible and repeatable effect, known as tenebrescence, is occasionally seen in natural materials, most notably hackmanite. Exposure to the light of an incandescent bulb faded the color faster, which is another property of tenebrescence. Exposure to short-wave UV did not produce any visible effect on the color.
The fact that hydrophane opal allows foreign substances to be introduced has already been established (N. Renfro and S. McClure, “Dyed purple hydrophane opal,” Winter 2011 G&G, pp. 260–270). This was in the context of dyeing with a substance that altered the appearance of the bodycolor, but did not change color in the presence of UV light. Impregnating opal with a UV-reactive chemical to make the opal appear to be tenebrescent has not been previously documented. Whether the substance used to impregnate the opals affected the original color or transparency is not known, nor is the stability of the treatment.