The Gemological Institute of America’s grading lab recently ran across a rare stone—a treated brown diamond—and it’s now warning there may be more out there.

“People need to be aware that brown stones could be treated,” says Wuyi Wang, the GIA’s director of research and development. “Don’t assume all browns are natural.”

The 1.04 ct. round brilliant had “unusual spectroscopic features,” according to Wang, and was created with a mixture of annealing and irradiation. More information can be seen in the Institute’s Gems & Gemology e-brief.

Wang says it’s difficult to specify the stone’s original color.

JCK