Gemological characteristics of sapphire from Hong Liem, Ham Thuan Bac district, Binh Thuan province, Viet Nam

 Basalt-related sapphire had been found in Pleistocene diluvial-diluvial deposits at Hong Liem, Ham Thuan Bac district, Binh Thuan province, Vietnam. The main color of the sapphire ranged from light to extremely dark bluish green to blue, and some are blue-yellow-green, yellow-green, and seldom yellow. Their original habits were barrel-shaped crystal and hexagonal tabula; transparency to opaque; from 2 to 10 mm in diameter, sometimes up to some cm. Some sapphire samples showed a typical trapiche sapphire. Their physical and optical properties belonged to the common sapphire range, inert under the UV lamp. Their common mineral inclusions included zircon, columbite, plagioclase, hercynite, and wüstite typical of mineral inclusions found in alkaline basalt-related sapphires.Their chemical characteristic is high Fe (0.5085–1.1364 wt%), Ti (0.0109 – 0.1042 wt%), with low contents of Cr (0.0037 – 0.0085 % wt). Their color was due to the presence of single Fe3+ at 376, 378 nm, Fe3+/Fe3+ pair at 450 nm and Fe2+/Ti4+ intervalence charge transfers (IVCT) with the maximum at 571, 579 nm, raising the absorption of Fe2+/Fe3+ IVCT towards the near-infrared (maximum approximately 870 to 890 nm). Magmatic sapphire’s source was recognized not only by the Cr2O3/Ga2O3 ratio lower than 1 but also by the structural O-H group at 3309 cm-1. This sapphire was in good quality, suitable for the jewelry of relatively high economic value.