Ch. 12: Topaz

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	124 A Book of Precious Stones

	refractive and dispersive, and its comparative softness makes its distinction from the diamond a simple matter. Although infusible, when sufficiently heated, the faces of crystallisation of topaz become covered with small blisters which crack as soon as formed; and with borax it slowly forms a clear glass. Some varieties assume a wine yellow or pink tinge when heated. The rose-pink topaz sometimes appearing mounted in jewelry, is not natural; the delicate tint of this gem with an artificial complexion results from a simple process called " pinking," applied to yellow or brown kinds. A topaz selected to be "pinked " is packed in magnesia, asbestos, or lime, and carefully and gradually heated to a low red heat; the stone then being slowly cooled. If the temperature attained has not been sufficiently high, the desired rose-petal tint is not obtained and a salmon tint appears; if the temperature rises too high, or is too long continued, the colour completely disappears. Pulverised topaz changes to green the blue solution of violets. Topaz generally becomes electric by heat, and if both terminations of the subject specimen are perfect, polarity will be developed; transparent varieties are susceptible to electrical excitation by friction.

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