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Heat-Treated Spinel

posted May 10, 2010, 7:28 AM by Alex Barcados
This 17.02 ct reddish orange oval modified brilliant was identified as a heated natural spinel. Photo by Jian Xin “Jae” Liao.
This spring, the New York laboratory examined a 17.02 ct reddish orange spinel that contained a few small expansion halos or "blebs" emanating from strings of minute inclusions. The appearance of the inclusions suggested the stone was of natural origin -- but may have been heat treated. 

Since experimenting on heat-treated Tanzanian spinels in 2005, the GIA Laboratory has tested several hundred other samples. Our studies have shown that unheated natural chromium-bearing spinel can be distinguished from synthetic or heated spinel through room-temperature photoluminescence (PL) spectroscopy. Chromium-related emission peaks in unheated natural spinel are usually very sharp. With heat treatment, these peaks become broad bands due to conversion of the crystal structure from "ordered" to "disordered." PL spectroscopy of this 17.02 ct stone showed broad chromium emission bands at 676, 687, 698, 708, and 717 nm. The PL data pointed to either a heat-treated natural spinel or a synthetic spinel.

Although the inclusions indicated that the 17.02 ct spinel was of natural origin, we confirmed this with laser ablation-inductively coupled plasma-mass spectrometry, which revealed significant amounts of various impurities (in contrast to the extremely low amounts in synthetic spinel).

Based on gemological, spectroscopic, and chemical testing, we identified this stone as natural spinel with "indications of heating." It is our understanding (through the gem trade and our own research) that heating may improve the clarity of some spinels, though not their color. The relative lack of inclusions suggests that the spinel we examined had been heated to improve its clarity.

- David Kondo, Riccardo Befi, and Donna Beaton
GIA Laboratory, New York