Geology, gemmological properties and preliminary heat treatment of gem-quality zircon from the central highlands of Vietnam

Abstract

Gem-quality brown zircon occurs in alluvial deposits derived from Neogene- Quaternary alkali basalts in the Central Highlands of Vietnam. Rough and cut samples were characterized using standard gemmological methods, chemical analysis (electron microprobe and LA-ICP-MS), and Raman, FTIR and UVVis- NIR spectroscopy. Among the internal features observed were colour zoning and various mineral inclusions, such as apatite, ilmenite, hematite and iron hydroxide. The analysed samples contained up to 1.14 wt.% HfO2 and 0.08 wt.% total REE oxides. Darker brown stones contained greater amounts of Hf than the lighter-coloured ones. Raman and FTIR spectroscopy showed that the zircon is high-type (i.e. has high crystallinity). As with the famous Ratanakiri zircon of neighbouring Cambodia, heat treatments can be applied to dark-coloured samples to lighten the coloration in an oxidizing atmosphere or induce blue colour in reducing conditions. © 2016 The Gemmological Association of Great Britain.