Observations on the Heat Treatment of Basalt-Related Blue Sapphires

This dark red crystal, presumably pyrochlore, in a Cambodian blue sapphire changed to red-orange with a surrounding partially healed fracture after low-temperature heat treatment. Darkfield and fiber-optic illumination. Photomicrograph by C. Khowpong © GIA, field of view 1.05 mm.

Basalt-related blue sapphires, which typically have a dark color when faceted, can be lightened through low-temperature heat treatment. This treatment is difficult to detect using standard gemological testing and microscopic examination. Researchers from GIA’s Bangkok laboratory sought an approach to identify this treatment. They performed spectroscopy (UV-Vis-NIR, FTIR, and Raman), LA-ICP-MS chemical analysis, microscopic examination, and fluorescence testing on 50 samples before and after heating.

The results are available in a study titled “Observations on the Heat Treatment of Basalt-Related Blue Sapphires” The authors determined that even with advanced testing, it is difficult to separate artificially heated basalt-related sapphires from natural unheated stones.