Many
crystals present beautiful changes of light, when this is reflected in
certain given directions, as in the Labradors, the moon-stone, and the
Asteria.
Minute crevices cause the iridescence, as in the stone called Iris.
Many
gems are more or less transparent ; that is, they possess in a greater
or less degree the power of transmitting light; and, according to these
degrees, they have different denominations. That stone is called "
transparent " which, when placed before an object, does not impede its
view.
When
the stone allows an object to be seen indistinctly through it, it is
called " semi-transparent," and " translucent " when it allows the
light to pass, but hinders the view of objects.
ELECTRICITY.
Electricity
is produced in gems by heat, friction, or violence. In some gems it is
caused by warming them at the fire, and then they are said to possess pyrelectri-eism. Electricity
is produced by friction in the topaz, the amethyst, all quartz, the
diamond, the garnet and emerald. Some crystals become electrical by
percussion, as, for instance, the Iceland spar.
Some stones conduct, others reject electricity, which in those cases is either positive or negative.
The power of retaining electricity when produced is very variable in stones, and the time which elapses
