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.
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