According to the American Gem Trade Association Gemological Testing Center (AGTA-GTC) + American Gemological Laboratories (AGL), a color-enhancing coating on tanzanite containing cobalt have been detected in faceted samples of tanzanites provided by their clients. Obviously the purpose of the treatement have been to improve the overall appearance of the stones, but with standard gemological testing + advanced analytical tools like X-ray fluorescence spectroscopy, the treatments are identifiable. Coatings are not considered permanent.
Useful links:
www.agta.org
www.agta-gtc.org
www.aglgemlab.com
www.ftc.gov
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