QUARTZ
This
is the most abundant of common minerals, and one which appears in a
great variety of colors and structures looking very unlike. In color,
hardness, transparency, and luster many of these varieties of quartz
are well suited for use as gems, but owing to their common occurrence
they are less highly valued than other minerals possessing perhaps no
more desirable qualities. Nevertheless the varieties of quartz have an
extensive use in jewelry, and deserve description in detail. The
chemical composition of all varieties of quartz is the same, viz.,
oxide of silicon. The physical characters are likewise nearly constant,
and are as follows: hardness, 7; specific gravity, 2.65; cleavage,
none; fracture, conchoidal; infusible before the blowpipe; insoluble in
common acids.
The
varieties of quartz fall naturally into two groups, the
pheno-crystalline (plainly crystalline), and cryptocrystalline
(obscurely crystalline). Of these the phenocrystalline varieties will
be considered first. These include rock crystal, amethyst, smoky
quartz, rose quartz, and sagenitic quartz, with others of minor
importance. The differences between these varieties are almost wholly
differences of color.
Rock Crystal. This
is quartz in its purest form. Typically it is transparent and
colorless, but clouded and opaque occurrences are included under this
head. By the ancients it was supposed to be petrified ice, and hence
the Greeks applied it to their word for ice, from which we get our word
crystal. One reason for this belief was the fact that much of the
quartz known to them came from the high peaks of the Alps. They
concluded therefore that it was ice frozen so hard it could not melt.
This belief must have survived nearly to modern times, for in 1676
Robert Boyle, the eminent physicist, thought it necessary to bring
forward several arguments to prove the falsity of the idea. One of
these arguments was that quartz was two and a half times as heavy as
water, and another that it was found in tropical countries.
Quartz
in the form of rock crystal is now known to occur in all parts of the
globe, although the occurrences of clear, transparent rock crystal
suitable for cutting are comparatively few in number. Rock crystal is
frequently, though not always, found in the form of terminated
crystals, having usually the shape of six-sided prisms capped at one or
both ends
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