THE QUARTZ FAMILY.
Quartz is a rock so plentifully distributed over the earth
that its claims to be ranked as a precious stone seem presumptuous ; but, as a fact, it yields a greater variety of gems of a
second class than any other mineral species, and has been
liberally used from time immemorial for engraving and jewelry.
Its hardness renders it susceptible of a good polish, and protects it from the consequences of ordinary attrition ; while its
great diversity of colors, and low prices, are favorable for its
general use in decoration.
The most common form of quartz crystal is hexagonal
prisms, which range from transparent to opaque, and display
nearly all the hues of the solar spectrum, diversified by numerous gradations. Quartz is infusible and insoluble, and possesses
double refraction and electrical powers ; its chemical constituent
is pure silica, except in the colored varieties, which contain
traces of certain other substances, as manganese, in amethyst,
nickel, in Chrysoprase, titania, in certain kinds of rose-quartz,
and manganese and iron, in many red, green, yellow, and brown
specimens.
The varieties are very numerous. Dana gives more than
twenty used for art purposes ; Church mentions a larger
number, but makes, in some instances, two varieties where
only slight differences exist, as cairngorm and smoky quartz,
heliotrope and bloodstone. Westropp arranges the different
349
