Opal is often treated to enhance its unique optical characteristics or improve its durability. Dye or impregnation methods in particular are easily applied to opals and other porous amorphous materials. A recent article on treated hydrophane opal (N. Renfro and S. McClure, “Dyed purple hydrophane opal,” Winter 2011 G&G, pp. 260–270) noted that we could expect to see more of this material. Two years later, this appears to be the case. The Lai Tai-An Gem Lab in Taipei received two fire opals for identification: a 1.09 ct cushion cut (7.67 x 7.64 x 4.67 mm; figure 1, left) and a 14.49 ct oval cabochon (23.80 x 15.27 x 9.12 mm; figure 1, right), both with pronounced play-of-color. Both gave RI readings of about 1.45. Observation with a gemological microscope and analysis with an FTIR spectrometer revealed the structure expected for natural opals. Orange color concentrations around some surface pits were also observed, indicating treatment. UV-visible spectra of natural and treated orange opals were also recorded for reference and comparison.
But the most obvious clue was the effect caused by simple immersion in water at room temperature overnight (figure 2). The water took on an orange tint, proving that the dye applied to these stones is water soluble, unlike the dye examined by Renfro and McClure (2011). This simple result reinforces the need for caution when buying this material. The change in color (shown in figure 3) would come as a shock to any client unfortunate enough to buy the material without suitable disclosure.
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