corundum, inclusions.5, Thailand.12, Chantaburi, star sapphire, gems

After a brief overview of some basic facts concerning asterism, the characteristics of black star sapphires from Thailand are described. As in star corundums from other sources, these stars were found to be diffraction phenomena created by needles or lamellae of rutile or hematite-ilmenite. These inclusions can be oriented either parallel or perpendicular to the second-order prism faces.. Six-rayed star sapphires from Bang-kha-cha [also spelled Bang Kacha] show bluish white or, less commonly, “golden” yellow stars. The bluish white stars are caused by Ti-bearing hematite (as are similar black stars from Australia), whereas the nature of the mineral(s) forming the yellow stars is unknown. In the bluish white stars, the band of light (chatoyancy) can be oriented both parallel and perpendicular to the second-order prisms, but they are always parallel to these prisms in the yellow stars.. Rarely, both colors of stars are combined in the same specimen and form 12-rayed star sapphires. The authors studied nine such sapphires and found that the bluish white stars were always oriented perpendicular to the prism faces, whereas the yellow stars were parallel to these faces. The article is illustrated with four excellent photographs and two diagrams, which make the rather complicated phenomena easily understandable.