Micro-World Gems & Gemology, Winter 2015, Vol. 51, No. 4

Violetish Blue Spinel in Yellow Sapphire

Violetish blue spinel inclusion in sapphire.
This violetish blue inclusion in a Sri Lankan yellow sapphire was identified as spinel. Photomicrograph by Nathan Renfro; field of view 5.63 mm.

Spinel as an inclusion in sapphire has been previously documented in blue and yellow sapphires from Sri Lanka (E.J. Gübelin and J.I. Koivula, Photoatlas of Inclusions in Gemstones, Vol. 1, ABC Edition, Zurich, 1986, pp. 353–354). Typically these inclusions are green zinc-rich ghanospinels. Recently the authors examined a transparent light yellow sapphire crystal with a prominent violetish blue octahedral inclusion. Interestingly, microscopic examination showed the violetish blue inclusion to be an intergrowth of at least two gemmy crystals, with the larger one crystallographically aligned with negative crystals that were also present in the host.

When further examined with polarized light, the violetish blue inclusion appeared to be singly refractive, which meant spinel was a possibility, considering their similar formation environments. The sample was polished down to a plate in order to place the violetish blue inclusion closer to the surface for Raman analysis, which conclusively confirmed that it was spinel. Also present in this sapphire were numerous carbon dioxide–filled negative crystals, fingerprints, and open cracks. The presence of the intact CO2 inclusions and the pristine condition of the spinel inclusion provided clear evidence that the sapphire host had not been subjected to any form of heat treatment. This is the first example of a violetish blue spinel inclusion in sapphire that the authors have examined.

Nathan Renfro is the analytical manager of the gem identification department and analytical microscopist in the inclusion research department, and John I. Koivula is the analytical microscopist, at GIA in Carlsbad, California.