A set of faceted spinel with blue to green hues, marked as “cobalt diffused,” was obtained by researchers from GIA in Bangkok. Analyses of the absorption, luminescence, and chemistry of the stones have revealed that they were not diffused with cobalt, but instead with nickel. Laser ablation–inductively coupled plasma–mass spectrometry indicated high nickel concentrations (tens of thousands of ppma) at the edge of the stones, and a compositional traverse across one spinel showed a decrease of nickel toward the core, consistent with diffusion treatment.
The trace element concentrations in the cores were consistent with natural spinel, as were the refractive indices. A similar decrease in lithium concentration was also seen, suggesting that treatment was done in the presence of a flux melt. Absorption spectra recorded in the ultraviolet/visible/near-infrared region contained characteristic bands at 369, 381, 475, 597, and 639 nm. Annealing experiments conducted on one spinel demonstrated that the 475 nm band is sensitive to the oxygen partial pressure (pO2) in the experiment. The band increases in intensity following a high-pO2 anneal, then decreases following an anneal at low pO2. Photoluminescence spectra were consistent with heated natural spinel.
This treatment now exists in the gem market and should be considered a possibility in spinel with blue or green hues.
Learn more about this topic by Michael Jollands, Abadie Ludlam, Aaron C. Palke, Wim Vertriest, Shiyun Jin, Pamela Cevallos, Sarah Arden, Elina Myagkaya, Ulrika D’Haenens-Johannson, Vararut Weeramongkhonlert, and Ziyin Sun as seen in Gems & Gemology, Summer 2023, Vol. 59, No. 2.
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