G&G: Tanzanite, other gems set with colored glue

A colored adhesive is present on the crown facets of this 1.87-carat tanzanite, shown here magnified 20 times. Photomicrograph by Nathan Renfro, (c) Gemological Institute of America (GIA). Reprinted by permission.

We publish courtesy of GIA‘s G&G

Carlsbad, Calif—The latest edition of G&G’s eBriefcontains several items of interest to those on the lookout for colored gemstone treatments: One is the discovery of tanzanite and other gems set with colored adhesive to darken their color, and another is the revelation that a ruby set in a piece of antique jewelry was actually filled with lead glass, a relatively new treatment.

The news was included in a G&G eBrief newsletter sent out Aug. 3 by Gems & Gemology, a Gemological Institute of America publication devoted to the latest gemological research.

The adhesive-set stones were discovered by a California goldsmith who was removing stones from some bezel-set rings that he had purchased in 2009 from a customer who had bought the rings from a TV home shopping network. He noticed that the tanzanite and other gems were set with what appeared to be colored adhesive, and alerted the GIA in March 2010.

Upon inspecting a 1.87-carat tanzanite submitted by the goldsmith, the GIA laboratory team found that a purple-colored flexible adhesive was visible on some of the crown facets, particularly at the corners. After it was removed, the color of the tanzanite appeared “very slightly lighter,” the eBrief said. The goldsmith told GIA that the other stones he removed from the rings became noticeably lighter once the adhesive was removed and that was particularly true for the amethyst.

“The colored adhesive was obviously intended to enhance the appearance of the stones, as well as help hold them in their mountings,” says the G & G eBrief item from GIA. “Buyer beware!”

The second colored stone treatment related item stemmed from a report from the American Gemological Laboratories (AGL), which uses the term “composite ruby” to distinguish lead glass-filled rubies from rubies that undergo the more common heat treatment. The former have drawn a lot of attention within the gemstone trade because they require special care during repairs and can also be damaged by common household chemicals.

The AGL item noted that despite the prevalence of the composite rubies in the marketplace, the lab was surprised to find such a ruby in an antique pendant submitted for identification. The pendant, which did not appear to be a replica, was set with old-mine-cut diamonds and seed pearls, consistent with its apparent age, but the lab identified the center stone as a composite ruby, with an estimated weight of 7.5 carats. The composite ruby had been carefully reset (the milgrain around the bezel was in good condition), and the lab detected no degradation of the glass in the stone—something that could be caused by the jeweler’s torch.

“The fact that this material has started to show up in antique jewelry is representative of how far it has penetrated the market and reinforces the importance of proper identification and disclosure,” the author of the eBrief item, from a staffer at the AGL laboratory, wrote.


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