Water Immersion Reveals Dye in Fire Opal
The Lai Tai-An Gem Lab in Taipei received two fire opals from a local dealer for identification: a 14.49 ct oval cabochon (23.80 × 15.27 × 9.12 mm; figure 1, left) and a 1.09 ct cushion cut (7.67 × 7.64 × 4.67 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/Raman spectrometer revealed the structure expected for natural opals. Orange color concentrations around some surface pits were also observed, indicating treatment.
But the most obvious clue was the effect caused by immersing the cabochon in water at room temperature overnight (figure 2). The water took on an orange tint, proving that the dye applied to these stones was water soluble, unlike the dye examined by Renfro and McClure (2011). The cabochon itself showed considerably less color after it was removed from the water (figure 24). This simple result reinforces the need for caution when buying fire opals, even ones with a natural-looking color. The change in color would come as a shock to any client unfortunate enough to purchase this dyed material without suitable disclosure.