AGTA GTC and AGL IDENTIFY COATED TANZANITE
NEW YORK, NY - 23 May 2008 – Recently the American Gem Trade Association Gemological Testing Center (AGTA GTC) and American Gemological Laboratories (AGL) received a number of faceted tanzanite samples that were determined to be coated. Evan Caplan of Omi Gems, Inc. sent samples to several labs after a light repolishing of a few stones resulted in a noticeable loss of color.
"Until now, we had not identified a coating on tanzanite to improve its color." indicated Dr. Lore Kiefert, Director of the AGTA GTC.
"Although the coating is not immediately obvious, careful examination with a microscope and in immersion provided clear indications of the coating in most instances." stated Christopher P. 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." Smith further added.
Advanced analytical testing identified that the coating was composed at least in part to cobalt. "The most reliable means to substantiate the presence of the coating is with the use of X-ray fluorescence spectroscopy." Kiefert explained "The coating is colored by cobalt, which is readily detected using this analytical technique."
Although the gemstone industry has become very familiar with the practice of heating zoisite to achieve the best violet to blue color in tanzanite, these stones represent the first time either lab has identified a coating on tanzanite to further improve its color.
The majority of the tanzanite sample was comprised of smaller calibrated stones. Fine color tanzanite in this size range is rarely sent to a lab and therefore would avoid detection unless closely scrutinized. "This is just a further reminder that each and every gemstone should be fully examined to determine whether or not it has been treated." Smith indicated "Today, it is not uncommon to see stones that have been treated using multiple or compound techniques to achieve a particular result."
As a closing statement, both Kiefert and Smith emphasized "Any treatment used to modify the color of a gem should be disclosed. Coatings in particular are not considered permanent and in the U.S. are required by FTC guidelines to be properly disclosed at the point of sale."
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Figure 1: These two tanzanites were part of a group of stones recently examined at both the AGTA GTC and AGL facilities in New York. Both were identified as having a thin coating layer to artificially improve their color. Photograph by Fred Kahn and Sun Joo Chung. |
Figure 2: The cobalt-coating produced a subtle iridescence that could be seen when observing the stone's surface in reflected light. The rectangular area near the point of this facet also highlights an area on the host tanzanite where the coating did not adhere. Photomicrograph by Christopher P. Smith |
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Figure 3: In immersion it was also noted that along facet junctions and at the culet the color was lighter. This was a result of where the coating had been abraded away, revealing the lighter, inherent color of the tanzanite. Photomicrograph by Christopher P. Smith |
