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34 PRECIOUS STONES.
when
a precious stone is breathed on its good conductivity causes the breath
to condense on its surface quickly and to quickly evaporate again,
whereas on a glass imitation both actions occur more slowly.
Fusibility.
Most gem stones fuse with difficulty, if at all, before the blowpipe ;
red Garnet, however, is moderately fusible, and where thin splinters of
the rough stones can be obtained this property may be made use of in
identification.
III. Electrical and Magnetic Properties.
A
surface charge of high potential electricity can be imparted to some
precious stones in a much greater degree than others. When produced by
rubbing with a dry cloth the charge is positive in cut gems, except in
the case of Amber, which becomes negatively electrified. The presence
of such a charge can be demonstrated by the electroscope or other
similar instrument. Most minerals soon lose their charge, even in dry
air, but Topaz, Sappbire and Diamond retain their charge for longer
periods, chiefly in the case of Topaz and least with the Diamond.
Electricity developed in uncut gems other than the Diamond is negative;
Calcite, Topaz, Fluor Spar and Quartz show electric phenomena on
pressure, especially Calcite.
When
some precious stones are beated, electricity is developed on them ;
this is known as pyroelectiicity. Axinite, Tourmaline and Topaz show it
well; a crystal of Tourmaline on heating to about 150° C. becomes
positively electrified at one termination and negatively at the other ;
if now it be suspended by a non-conducting thread it will
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