“Sugar Plum” in Titanium-Diffused Blue Sapphire

Taku Okada

A recently examined 4.20 ct blue sapphire included a frosted cluster surrounded by a rimmed bubbly discoidal fingerprint, resembling a sugar plum, Portuguese confeito, or Japanese konpeito (see above). The inclusion appeared to be a former zircon (ZrSiO₄) cluster, which is a common inclusion in sapphire and decomposes after heat treatment above about 1400°C (W. Wang et al., “The effects of heat treatment on zircon inclusions in Madagascar sapphires,” Summer 2006 G&G, pp. 134–150). When viewed with diffused white light, the stone displayed concentrations of blue color at the facet edges of the pavilion, indicating titanium diffusion treatment, which usually happens around 1675–1750°C for 96–150 hours (R.W. Hughes et al., Ruby & Sapphire: A Gemologist’s Guide, RWH Publishing/Lotus Publishing, Bangkok, 2017). At such high temperatures, the aluminum oxide (Al₂O₃) component of the surrounding corundum decreases the melting point of the decomposing zircon and causes partial melting, especially through interaction with the SiO₂ component of the zircon inclusion (W. Wang et al., 2006). High-temperature heat treatment sometimes creates such sugar plum–like frosted zircon inclusions as seen in this sapphire.