Heat treatment of basalt-related sapphire from Binh Thuan deposit, Southern Vietnam

Characteristics of basalt-related sapphires from Binh Thuan in Southern Vietnam were investigated before and after heat treatment. Heating at 700℃ to 1000℃, caused blue hue to fade noticeably, while blue color was intensified when heated at 1650℃. Subsequently, these sapphire samples were subjected to beryllium (Be)-assisted heating by a local professional gem treater, which yielded yellow to yellowish-blue hues. Absorption spectra of blue sapphires indicated absorptions at 378 nm, 387 nm, 450 nm, 580 nm, and 880 nm. However, yellow sapphires, after Be-assisted heating, did not exhibit absorptions at 580 nm and 880 nm. In the FTIR spectra, bands associated with OH stretching disappeared after being heated to 1650℃ as well as Be-assisted heating. Based on LA-ICP-MS analyses, an increase in Be concentration (5 ppma to 146 ppma) was detected following Be-assisted heating, which conclusively induces yellow coloration in sapphire.