For green gems, we have the emerald, Chrysoprase, tourmaline, peridot, garnet of the Urals, aquamarine, and beryl.
Blue stones comprise the sapphire, spinel, iolite, lapislazuli, and indicolite (a variety of tourmaline) ; while for
purple, the amethyst and the almandine garnet afford
examples.
The color of some precious stones, when looked at, is
different from that seen when looking through the crystal ;.
that is, their reflected rays are not like those transmitted,
as is the case with the tourmaline and the sapphire d'eaur
or iolite.
Dichroism (the quality of exhibiting two colors) and pleocliroism (the quality of exhibiting more than two) are, it is supposed, due to the refraction and polarization of light ; and it is
only in double-refracting crystals that these properties inhere
The different colors displayed by these minerals depend upon
the direction in which they are viewed.
The tourmaline affords the best illustration of this remark
able property, presenting, as it does, red, yellow, green, and
blue, with some other tints, in the same crystal ; the emerald
and the ruby exhibit the same phenomenon, only in a less
degree. A blue sapphire, examined by a dichroscope, affords,
besides its ordinary color, a greenish yellow, the topaz a pink
and yellow ; while andalusite, from Brazil, gives white, green,
and pink hues.
LiÎStre, an important quality in gems, depends upon their
structure, texture, and reflecting powers. There are different
kinds of lustre, as adamantine, like the diamond ; vitreous, like
glass ; resinous, pearly, and silky. The brilliancy of a precious
stone is the result of its lustre, or its power of receiving a
polish, and may differ in the same species.
Transparency.—The capacity of transmitting light enhances
