INFLUENCE OF HEAT TO SILKY-MINERAL INCLUSION IN SAPPHIRE FROM CHANGWAT CHANTHABURI

Opaque sapphires with brownish color have low demand in the gems market. These opaque sapphire contains by abundant silky inclusions. These inclusions may produce special phenomenon of asterism in star sapphire. Sapphires with silky inclusion from Bang Kacha area, Changwat Chanthaburi were sampled for this study. Samples were divided into three groups consisting of minor silky inclusions; moderate silky inclusions and abundant silky inclusions. Large amount of silky inclusions features can be observed in moderate silky inclusions and abundant silky inclusions groups under gemological microscope. These inclusions were identified by Raman microscope and were also analyzed chemically using EPMA. The results from both techniques indicate that the mineral inclusions are hematite. Transparency of the samples were improved after heat treatment at 1650 ºC for 3 hours. Zones of silky inclusions with grey-brown color were turned to blue after heating. The UV-Vis-NIR spectrum of heated samples showed strong absorption at about 900 nm due to Fe2+/Fe3+ IVCT which conclusively indicated main color-causing of blue.