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.