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PRECIOUS STONES
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turbing
foreign element within them, producing abnormal conditions. The three
isometric stones, diamond, spinel, and garnet, are normally, like
glass, singly refractive.
Dispersion
is the power which decomposes a ray of common white light in its
passage through a transparent medium, and splits it up into the various
colors of which it is composed. This power is very high in the
diamond, and gives to it the fire which makes the stone so fascinating.
By holding a diamond to receive the direct rays of the sun, and a
sheet of white paper at an angle to catch the reflected light from it,
the prismatic colors will appear.
The
lustre of the various stones is differently described. Most of the
transparent are vitreous. The diamond and zircon are adamantine. The
hematite is metallic. Of the opaque, translucent, and semitransparent,
some are said to be waxy, as the turquoise; silky, as the crocidolite;
pearly, as the moonstone; subvitreous or glassy, as the opal.
It
is a curious fact that the powder of a mineral does not always
correspond with it in color. The color of its powder is called the
streak-powder, and the color of the surface from which it was abraded,
the streak. The streak of the ruby is white; of the diamond, gray or
blackish gray. That of most colored stones is uncolored.
One
of the distinguishing differences between stones is the weight relative
to the bulk. A ruby of the same bulk as an emerald would weigh more, a
zircon would be still heavier, and a microlite or cassiterite heavier
yet. This relation is found by comparing the weight of objects with
that of another substance containing the same volume of matter, and is
called specific gravity. For instance, if a stone weighing fifteen
carats, upon being weighed in distilled water only weighed ten carats,
its specific weight would be ten carats, and the loss of weight, five
carats, would be the weight of the volume of water displaced by the
stone and equal to its bulk. Dividing the absolute weight of fifteen
carats by the loss by
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