Gemological characteristics and origin of basalt-related sapphire in Dong Nai province

Abstract

Basalt – related sapphire in Thong Nhat District, Dong Nai Province, Vietnam, are found in alkaline basalts and their lateritic weathering layers. This study employs standard gemological methods and advanced analytical techniques such as XRD, EDX, Raman spectroscopy, and FTIR to characterize their gemological properties, crystal chemistry, and formation origins. The results indicate that the basalt-related sapphires in Dong Nai has the chemical formula Al₂O₃, crystallized in the hexagonal system. They exhibit a distinctive morphology with diverse colors ranging from deep blue to green and pale yellow, depending on Fe and Ti contents and color centers. Identify properties include a specific gravity of 3.97–4.03, refractive indices of 1.768–1.778, and birefringence of 0.008–0.012. They are non-fluorescent under UV light due to high Fe content, with characteristic inclusions of rutile, hematite, and zircon. Raman and FTIR spectra suggest these sapphires are basalt-related in origin. The inclusion characteristics of zircons and their association with sapphires are similar to those of zircons from syenite. Additionally, the amount of isotope oxygen-18 falls within the range typical of syenite, confirming that the sapphire originated from syenitic magma.. The Dong Nai sapphires are of medium to low quality, suitable for use in jewelry as faceted stones or cabochons. Notably, star effects and trapiche structures enhance their appeal and significantly increase their value in the study area. This research aims to provide detailed data on the gemological characteristics, origins, and potential for sapphire exploitation in Dong Nai Province.