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Ch. 26: Topaz

Ch. 25: Spodumene Page of 252 Ch. 26: Topaz Text size:minus plus Restore normal size   Mail page  Print this page

Remarkable clearness and transparency, capacity of taking a high polish, and hardness and weight greater than that of quartz are the qualities in which topaz excels as a gem. Numerous other stones of inferior quality masquerade under its name, however, and this fact may account for the decline in popularity which the stone has suffered in recent years. True topaz is a silicate of alumina, containing hydroxy! and fluorine. Its hardness is 8, and it thus scratches quartz. Topaz is also remarkably heavy, considering its composition, it being three and one-half times as heavy as water. Owing to this unusual specific gravity, those accustomed to handling gems can frequently pick out the topaz from a miscellaneous lot of precious stones without remov­ing their wrappings.
The color typically associated with topaz in its use as a gem is yellow. Yet the mineral species exhibits many other shades of color, which, when present in crystals of sufficient clearness and purity, answer equally well for gem purposes. These other shades, most of which are repre­sented in the accompanying plate, are grayish, greenish, bluish, and reddish. Topaz may also be quite colorless. The yellow color of the Brazilian topaz can be changed by heating to a pale rose-pink, and the gem is often treated in this way. The degree of heat employed is not high, and both heating and cooling must be performed gradu­ally. The selected stone may be packed in magnesia, asbestos, or lime, and heated to a low, red heat, or it may be wrapped in German tinder and the latter set on fire. Only stones of a brown-yellow color yield the pink; the pale yellow stones turn white when so treated. Once the pink color is obtained it is permanent. The natural colors of topaz are in general perfectly durable, although some of the deep wine-yellow topazes from Russia fade on exposure to daylight.
Topaz is infusible before the blowpipe. It is not affected by hydro­chloric acid; but is partially decomposed by sulphuric acid, and then yields hydrofluoric acid. If the latter experiment is tried in a closed glass tube, the formation of the hydrofluoric acid is made evident by the etching and clouding of the walls of the tube. The powdered stone should be mixed with acid sulphate of potash for this experiment.

Ch. 25: Spodumene Page of 252 Ch. 26: Topaz
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