Mineral Guide > Precious Stones > Topaz gem stone

Topaz gem stone

Topaz gem stoneRemarkable clearness and transparency, capacity of taking a high polish, and hardness and weight greater than that of quartz. These are the qualities in which Topaz excels as a gem. True topaz is a silicate of alumina, containing hydroxyl and fluorine. Its hardness is 8 in the scale in which quartz is 7. Hence, it will scratch the latter mineral and may thus be distinguished from it. It is also remarkably heavy, considering its composition, it being three and one-half times as heavy as water, while quartz is only two and one-half times as heavy.

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. Warming in a sand bath at a low red heat is the method usually employed, or the stone may be wrapped in German tinder and the latter set on fire. Only stones of a brownyellow 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.

The crystals of topaz belong to the orthorhombic system of crystallization. They are usually elongated in the direction of the prism and have sharp, bright faces. They vary much in size and often are large. One crystal weighing twenty-five pounds was found in Siberia.

Waterworn Topaz picture