Gem News International Gems & Gemology, Fall 2013, Vol. 49, No. 3

Water Immersion Reveals Dye in Fire Opal

Figure 1. These fire opals, a 14.49 ct cabochon (left) and a 1.09 ct cushion cut (right), are shown prior to immersion in plain water at room temperature. Note the natural-looking color. Photos by Larry Tai-An Lai.
Opal is often treated to enhance its play-of-color 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 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.

IMG-FA13-GNI 154353 636x358
Figure 2. The clear water in this glass beaker (left) turned orange after soaking the fire opal cabochon overnight at room temperature (right). The obvious water dis¬col¬oration proves the sample was dyed. Photos by Larry Tai-An Lai.
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.

IMG-FA13-GNI 154351 636x358
Figure 3. After removal from the water, this cabochon exhibited an obvious loss of color, indicating dye treatment. Photo by Larry Tai-An Lai.

Larry Tai-An Lai operates the Lai Tai-An Gem Laboratory in Taiwan.