REVERSIBLE COLOR MODIFICATION OF BLUE ZIRCON BY LONG WAVE ULTRAVIOLET RADIATION

Blue zircon has prominent use as a gemstone in the jewelry industry where it is prized for its brilliance, vibrant color and high clarity. The material exists as crystals of various colors from a number of countries. The brown material from Cambodia must be heat treated to ~1100¢ª C in a reducing atmosphere to produce a generally stable, desirable blue color. However, when this heated blue zircon is exposed to long-wave ultraviolet irradiation, a brown coloration is introduced which in turn results in a much less attractive and much less valuable gemstone. Common sources of accidental long wave irradiation that can affect faceted blue zircons set in jewelry are tanning beds, as well as UV lights for curing epoxies used in the adhesion of artificial acrylic fingernails. To determine if the brown color in zircon induced by LWUV irradiation is completely and easily reversible, quantitative ultraviolet-visible spectroscopy was used to measure the difference in absorption before and after irradiation treatment. The goals of this study were to explore the nature of the UV-induced brown color-causing defect, and determine if subsequently exposing the stone to visible light would completely restore the blue color. Twelve samples were examined for this study, and spectroscopy showed that the blue color is due to a broad absorption band in the ordinary ray starting at 500 nm and centered at approximately 650 nm. LWUV exposure induced an absorption feature centered at approximately 490 nm, which was responsible for the brown color. The 490-nm absorption was completely removed by exposing the material to broad spectrum visible light for approximately one hour.