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ELECTRICAL PROPERTIES, PHOSPHORESCENCE, AND FLUORESCENCE
All
gems when rubbed upon cloth become, like glass, positively electrified.
Gems differ, however, in the length of time during which they will
retain an electrical charge. Thus tourmaline and topaz remain electric
under favorable conditions for several hours; but diamond loses its
electricity within half an hour. The electrical peculiarities of
different species were at one time used quite extensively for
identifying them; but owing to different behavior under different
atmospheric conditions little use is now made of such tests. Besides
developing electricity by friction some gems become electric upon
heating. Such are said to be pyro-electric. To test a stone, or rough
piece of mineral for this property, it can conveniently be held in
forceps and heated gently in a colorless flame, such as that of a
Bunsen burner or alcohol lamp. The amount of heating should not be much
over 100° C. On withdrawing the stone, it will, as it cools, if
pyro-electric, attract bits of tissue paper or straws. Tourmaline is
perhaps the most strongly pyro-electric of the minerals used as gems,
and the property affords a means for identifying it. Topaz is another
gem mineral which usually exhibits this property. Some topaz also
becomes electric when subjected to simple pressure. This is said to be
true of some crystals of Brazilian topaz if they are pressed between
the fingers in the direction of the vertical axis. Electricity
developed in this way is known as piezo-electricity.
Simple
tests for all these kinds of electricity consist in the attraction of
light objects, such as bits of tissue paper, cat hairs, pith balls
suspended by silk threads, etc. They are best made when the atmosphere
is dry, the winter season being especially favorable.
Some
gems have the property of emitting light after heating, exposure to
light, or an electrical discharge. This property is known as
phosphorescence, since the glow, although it is often of different
colors, resembles that emitted by phosphorus. The diamond is a mineral
which exhibits this property, some of its gems after exposure to
sunlight for a short time emitting a glow which can be plainly seen in
a dark room. This is often stated to be a property of all diamond, but
this is incorrect,
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