Cobalt-Colored Composite Sapphires
Fine needles of rutile were also observed in the natural sapphire component of these composite sapphires. Photomicrograph by Christopher P. Smith, 72x.
The present description of cobalt-colored, lead-glass treated sapphire expands on a press release issued by the American Gemological Laboratories (AGL) in January 2013. In May 2012, AGL first learned of a “new” type of treated sapphire which was becoming available in the Bangkok market. At that time this treatment was described as a new type of lead-glass treated corundum where a blue colorant was also added.
Upon testing a select number of samples, these stones were found to be a composite of natural, low-grade corundum and a high lead-content glass, where cobalt has been added to the composition of the glass in order to induce a blue coloration. As a result, these composite stones involve both a clarity and color treatment. What follows is a description of the geological features that allow these stones to be identified and a discussion on their disclosure nomenclature.
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