The Gemological Institute of America (GIA) recently discovered that a synthetic blue sapphire can change its color permanently when exposed to ultraviolet light.
A customer brought in a 1.70-carat, light blue gemstone to the GIA’s laboratory in New York, believing it was an alexandrite. The examination included standard gemological testing, which revealed the stone was a synthetic sapphire that reacted when exposed to ultraviolet light, according to an article in the Spring issue of Gems & Gemology, the institute’s quarterly journal.
The lab initially looked at the stone under a microscope, which showed it was internally clean with no display of curved colors under diffused light. Due to the lack of diagnostic inclusions and growth structure, the GIA carried out further testing under shortwave ultraviolet light. The brief exposure, lasting only seconds, triggered a photochromic shift in which the color of the stone changed from light blue to yellow. Elevated nickel concentration may have contributed to the rapid and significant color change observed in the stone.
Photochromism — a reversible color change after exposure to electromagnetic radiation — resulting in yellow and orange hues has long been recognized in sapphire. However, previously, any color change ultraviolet light has induced was undone through exposing the stone to white light. The GIA examiner tried to reverse the process by carrying out various tests, including a high-strength incandescent light source, X-ray exposure and long-term illumination under LED light. Absent any success, the lab returned the original light-blue synthetic sapphire to the customer as a yellow stone.
“To the authors’ knowledge, this is the first documented case of corundum with irreversible photochromism encountered in a gemological laboratory,” the GIA noted. “One should avoid exposing these pale-blue laboratory-grown sapphires to UV radiation until this phenomenon is better understood.”
Image: The synthetic sapphire in before and after the color change. (Gemological Institute of America)
