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Tanzanite Coating Identified
Jewelry Scene

Tanzanite joined the big leagues this summer: Two gem labs announced that they had tested samples that had been coated to improve their color. The American Gem Trade Association Gemological Testing Center and American Gemological Laboratories sent out a joint statement announcing the new tanzanite coating.

“Until now, we had not identified a coating on tanzanite to improve its color,” says Dr. Lore Kiefert, director of the AGTA lab. Tanzanite is routinely heat treated to create its deep blue to purple color.

The coating, on a parcel of small calibrated stones, was first detected by Evan Caplan of Omi Gems, Los Angeles, who sent samples to several labs after a light repolishing of a few stones resulted in a noticeable loss of color.

“Although the coating is not immediately obvious, careful examination with a microscope and in immersion provided clear indications of the coating in most instances,” explains Christopher Smith, vice president and chief gemologist of AGL. “This was evidenced by abrasions along facet junctions and at the culet where the coating had worn off, as well as a subtle iridescence when viewing the surface with reflected light.”

Advanced testing identified that the coating includes cobalt. “The most reliable means to substantiate the presence of the coating is with the use of x-ray fluorescence spectroscopy,” says Kiefert.