Development of Green Colour in Topaz Using Diffusion Method
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Date
2021
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Uva Wellassa University of Sri Lanka
Abstract
Topaz is one of the abundant gem varieties in Sri Lanka with the chemical formula of Al2SiO4(F,
OH)2, which has a hardness of 8 on the Moh’s hardness scale. The colourless Topaz varieties have
less market value compared to the coloured Topaz varieties. Irradiation and other methods are used
traditionally to improve the appearance of Topaz. Nevertheless, conventional methods have
drawbacks such as high manufacturing cost, less operational safety due to radioactivity related
processes. Hence, the present study tested the diffusion effect of Chromium (II) chloride on
colourless topaz with the aid of heat treatment. Seventy-two colourless Topaz samples were treated
with Chromium (II) chloride. The selected stones were heated under reduced and oxidizing
conditions with the Chromium (II) chloride at different temperatures (600 0C, 700 0C, 800 0C, and
900 0C ) and different soaking times (1, 2, and 3 hours). In both reduced and oxidizing conditions
Chromium (II) chloride, diffused into the topaz by producing a shade of pale green to dark green
colour. Colour shade becomes strong with the increase of the temperature. Produced colours created a
desirable appearance on the stones. At 900 0C the produced green colour was identified as ‘GB/BG
7/2’ in the GIA colour grading system. The durability of the coating was tested for acidic conditions
(with HNO3 and H2SO4 acids) and sonication. The produced surface coating was stable under acidic
conditions as well as under the sonication. Oxidizing conditions in electrically operated furnace is
economically beneficial over gas furnace in producing colours. Finally, colourless Topaz could
develop the green colour by diffusion with the optimum temperature of 900 0C with the soaking time
of 1 hour under oxidizing conditions.
Keywords: Diffusion Treatment; Topaz Enhancement; Colour development of gemstones
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Keywords
Mineral Sciences, Materials Sciences, Gemstones, Applied Earth Sciences, Geochemistry