A Zircon with Strong Photochromic Effect
Recently, a 6.54 ct oval faceted gemstone with greenish blue color (figure 1, left) was sent to Taiwan Union Lab of Gem Research (TULAB) for identification. The specific gravity of this stone was 4.68, and the refractive index was over the limit of the refractometer. Microscopic observation showed strong birefringence. In addition to standard gemological testing, Raman spectroscopy and comparison with the zircon reference spectrum R050203 from the RRUFF database (figure 2) confirmed it was a zircon. It was particularly worth noting that this zircon showed a significant color change from greenish blue to very dark yellowish green (figure 1) when exposed to a long-wave ultraviolet lamp.
To determine the extent of the color change and whether it was permanent or reversible, the zircon was first exposed to long-wave UV light for one minute and then under 10W white LED light for another minute (the light sources were placed approximately 3 cm away from the gemstone). After repeating this process several times with each exposure one minute longer than the previous time, we confirmed that the color changed from medium light greenish blue with strong saturation to a medium dark greenish yellow with lower saturation after two minutes of long-wave UV light exposure. However, the greenish yellow color gradually returned to the stable greenish blue color after photobleaching with LED white light for 30 minutes. Therefore, the stone was a photochromic zircon with reversible color change (as reported in N.D. Renfro, “Reversible color modification of blue zircon by long-wave ultraviolet radiation,” Fall 2016 G&G, pp. 246–251). After the color change reached its full extent under long-wave UV light, the zircon was analyzed by visible spectroscopy to record its continuous spectral change during the photobleaching process every six minutes (figure 3). The resulting spectra revealed that the light transmittance in the range between 450 nm and 550 nm gradually increased during the photobleaching process, and the greenish blue color finally returned to a stable state after 30 minutes.
Although this type of photochromic zircon has previously been reported, a zircon over 6 ct with such a significant photochromic effect is still rare and worth noting, especially since it exhibited a distinct difference in hue, tone, and saturation.