Microscopic Observations of Blue Sapphires Treated with Heat and Pressure
In recent months, sapphires treated under heat and pressure have been documented in the trade. This material is reported to be treated under pressure of 1 kbar at temperatures between 1200 and 1800°C by HB Laboratory Co. Ltd. of Korea (H. Choi et al., “Sri Lankan sapphire enhanced by heat with pressure,” The Journal of the Gemmological Association of Hong Kong, Vol. 39, 2018, pp. 16–25). This process is used to improve blue coloration and potentially heal any fractures within the stones.
GIA acquired from Sri Lankan gem dealer Imam Faris of Imam Gems (Pvt) Ltd. six samples that were said to have been treated under the conditions mentioned above. The samples were examined using a standard gemological microscope to look for any unusual features resulting from this type of treatment. Five of the six sapphires showed a feature that to our knowledge has not been reported before. Viewed in polarized light (figure 1, left), planes of small crystallites were observed as bright spots against a dark background when the host corundum was in the extinct position. But under unpolarized light (figure 1, right), the crystallites were either invisible or nearly so, and only visible from a low-relief interface between the crystallite and the host corundum. Due to the extremely low relief of these inclusions, the author speculates that these inclusions are corundum crystals that originated from fractures and were healed by recrystallization of the corundum. While it is certainly possible to have birefringent crystals of corundum in untreated corundum, the consistent observation of the planes of birefringent crystals by the author suggests that this feature could be a useful indicator for corundum treated with heat and pressure. These preliminary observations offer some interesting clues, but further research will be needed to fully characterize corundum treated with this technique.