gems, spinel, synthetic spinel, spinel treatments, heat treatment, spinel diffusion, nickel diffusion

A group of faceted spinels with blue to green hues, initially labeled as “cobalt diffused,” was examined by GIA researchers in Bangkok. Detailed analysis of their absorption, luminescence, and chemical composition revealed that these spinels were in fact diffused with nickel, not cobalt. Laser ablation–ICP–MS showed nickel concentrations in the tens of thousands of ppma near the surface, decreasing toward the core—confirming a diffusion profile. The trace element composition at the cores was consistent with untreated natural spinel, and refractive indices also matched those of natural specimens. A simultaneous drop in lithium content across the diffusion gradient suggested the use of a flux-assisted diffusion process. UV–VIS–NIR spectra showed characteristic absorption bands at 369, 381, 475, 597, and 639 nm. The 475 nm band was particularly responsive to annealing under different oxygen partial pressures, increasing under high-pO₂ and decreasing under low-pO₂ conditions. Photoluminescence spectra were typical of heat-treated natural spinel. This nickel-diffusion treatment is now present in the gem market and should be considered when evaluating blue or green spinel for potential enhancement.