In June, the German Gemmological Association celebrated its 75th anniversary, hosting Europe’s first International Gemmological Symposium, entitled Gemmology: The Present and The Future. Gem expert and author Antoinette Matlins was there to report the highlights.
One hot topic was the increasing use of coatings on diamonds, gemstones, and pearls. “Dr. James Shigley, from GIA, told of the latest findings related to surface coatings used on diamonds, and stressed that coatings are being used increasingly on other gemstones and pearls,” Matlins says. “For diamonds, coatings are used to create fancy colors and to mask the tint of color, thereby creating ‘colorless’ and ‘near colorless’ diamonds. In colored gemstones and pearls, coatings are being used to alter or improve color, overall appearance, and durability.
“Detection of surface color-coatings is not difficult and often requires nothing more than a loupe or microscope, but Shigley warned that colorless coatings can present challenges.”
Matlins continues, “Recent research findings on diamonds reported to have been subjected to diffusion techniques were found not to have been diffusion techniques at all, but also the result of surface coatings. Shigley warned that there are an increasing number of products in the market with deceptive claims, such as those claiming to be coated with diamond when in fact they are coated with DLC (diamondlike carbon). Shigley pointed out that DLC is not diamond, nor does it have diamond’s physical characteristics. He pointed out that calling such products diamond coated is fraud.
“Chris Smith from AGL reported on blue sapphires that were reportedly created by a new beryllium-diffusion treatment, but testing revealed that the color was another example of surface coating, this time using cobalt. As other coatings, this one is easily detected with standard gemological testing.”
Matlins asked Smith if anyone had used a Chelsea filter to test for cobalt. No one had. Matlins suspects that these sapphires will reveal a strong red reaction when viewed with the Chelsea filter. If she’s right, the Chelsea filter would give an immediate indication that something is wrong, and if other testing indicates sapphire, then you would quickly know its surface was coated with cobalt.