Tasnara Sripoonjan

Chanthaburi, eastern Thailand, is one of the world-renowned gemstones producers. It is designated as a “City of Gems” as most of the world’s gemstones pass through the city. Chanthaburi has long been known for high-quality gemstones derived from basalt-related deposits (e.g., Bang Ka Cha, Khao Wua, Khao Ploi Waen), especially natural yellow sapphire–Mekong Whisky color–which is unparallelly unique. The important studies (e.g., Hughes, 2002; Emmett et al., 2003) previously found that an application of lattice-diffusion heat treatment with beryllium could predominantly produce attractive colors, such as yellow, yellowish orange, in many types of sapphire. This made unmarketable sapphires could be modified to a characteristic golden-yellow color and used in fine jewelry (Figure 1). However, the supply of those raw materials has been substantially reduced and is presently deficient. The semitranslucent-opaque sapphires (Figure 1) have been discovered to substitute the original unmarketable ones, otherwise, they were often cut as a cabochon to exhibit dark color with asterism, aka black star sapphires. The dark body color, as well as almost opaque transparency, is characterized by high Fe-rich content and densely exsolved silks of hematite and/or in part other Fe-Ti oxides (Saminpanya, 2001; Sripoonjan et al., 2013, 2014) resulting in poor diaphaneity of these sapphires. In recent years, the proliferation of new treatments provided great potential for improving gemstone quality, the less valuable sapphire can potentially turn into intense and stable yellow sapphire applying beryllium-assisting treatment (Be-heating). This study aims to carry on an experiment of lower quality semitranslucent-opaque sapphires treatment and to investigate the cause of changes in color and clarity.