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Meet the Experts cobalt coated tanzanites


Cobalt Coated Tanzanites

cobalt tanzanitesAnchorCert detected examples of tanzanites which had been enhanced by the latest process – cobalt coating. The treatment takes the form of a surface coating which can improve the apparent colour of the tanzanite by as much as two grades, ie from A or AA to AAA or AAAA. The colour enhancement makes the stone apparently better quality, and therefore more valuable, than it was originally. However, the cobalt can be polished off and the treatment should definitely be disclosed by the seller.

The treated stones detected by AnchorCert were between 30 points and one carat and the supplier knew that the stones would be submitted for certification. This suggests that some of the supply chain is buying and selling in good faith and is not yet aware of the new treatment, which only became available commercially within the last 12 months. The cobalt treatment is sometimes quite hard to identify with a 10x loupe, but under higher magnification, using a gemmological microscope there are give away signs. AnchorCert gemologists automatically look for the signs and if the coating is in any way imperfect they are easy to spot.

On a perfect example the only way you can tell with the naked eye is by the colour, which just doesn’t look right. When checked with a loupe you can see that the colour is patchy, particularly when viewed from the crown, and when the stone is put under X –Ray Fluorescence it will show relatively high levels of cobalt.


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