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30 PRECIOUS STONES
(f)
Metallic or Sub-metallic. The former when the lustre is perfect as in
gold ; the latter when the stones possess the less true lustre of
copper.
Colour.—Colour
is an effect entirely dependent upon light, for in the total absence of
light, such as in black darkness, objects are altogether invisible to
the normal human eve. In daylight, also, certain objects reflect so few
vibrations of light, or none, that they appear grey, black, or
jet-black : whilst those which reflect all the rays of which light is
composed, and in the same number of vibrations, appear white. Between
these two extremes of none and all we find a wonderful
play and variety of colour, as some gems allow the red rays only to
pass and therefore appear red : others allow the blue rays only and
these appear blue, and so on. through all the shades, combinations and
varieties of the colours of which light is composed, as revealed by the
prism. But this is so important a matter that it demands a chapter to
itself.
The third physical property of light, Phosphorescence, is
the property possessed by certain gems and minerals of becoming
phosphorescent on being rubbed, or on having their temperature raised
by this or other means.
It
is difficult to say exactly whether this is due to the heat, the
friction, or to electricity. Perhaps two or all of these may be the
cause, for electricity is developed in some gems—such as the topaz—by
heat, and heat by electricity, and phosphorescence developed by both.
For
example, if we rub together some pulverised fluorspar in the dark, or
raise its temperature by the direct application of heat, such as from a
hot or warm iron, or
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