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Renewed interest in large-scale commercial mining of Montana sapphires motivated a detailed study of the heat treatment of sapphire from the Rock Creek deposit. During the course of this systematic study, over 75,000 stones were processed. It was found that approximately 65%-70% of the pale blue, pale green and near-colorless sapphire could be converted to well-saturated blue and yellow colors. Some improvement in clarity was noted as well. Yellow hues were easily developed by heating in oxidizing atmospheres and were relatively insensitive to time and temperature. Blue hues were developed by processing in reducing atmospheres. The relatively high iron content of Rock Creek material restricts the range of reducing conditions that can be used to achieve optimal coloration because of hercynite precipitation in the sapphire.
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Gems & Gemology
Summer 2017 issue