The quartz group is the largest and most diversified among precious
stones. Quartz occurs _massive_, in concretions, and in confused
crystalline masses.
On account of the abundance of the massive kinds, such as jasper,
agates, onyx, etc., some writers place the quartz group under the
head of semi-precious stones, and lately the United States customs
authorities have gone further in that direction, and have ruled that
“because of the abundance and comparative cheapness of agates, onyxes,
etc., they were no longer precious stones.” This position, however, the
custom-house speedily abandoned, and, for dutiable purposes at least,
the quartz family, in all its ramifications, is recognized as belonging
to the precious stones.
Harder than the tourmaline, turquois, or opal, as hard as the
chrysolite, and nearly as hard as the garnet or emerald, there is no
reason why the crystallized varieties, such as amethyst, cairngorm,
false topaz, chrysoprase, and even the cat’s-eye and finer onyxes,
should not be classed among the precious stones.
Some more plentiful and less beautiful varieties of quartz are not
valuable, and they take the same position in the quartz family that the
huge imperfect crystals do in the beryl group. Whenever the specimen is
sufficiently beautiful to be cut and polished for setting in jewelry,
it should be included under the precious stones.
Quartz crystallizes in the rhombohedral system, and many varieties are
found massive and compact. The cleavage is indistinct but can sometimes
be found by plunging a heated crystal into cold water. The hardness of
quartz is 7; specific gravity 2.5 to 2.8, the purest kinds being 2.65;
the lustre is vitreous to resinous, and fracture conchoidal.
Quartz is tough, brittle, and feels cold; it becomes positively
electric by rubbing, shows phosphorescence in the dark, and gives
sparks if struck with another piece of quartz or with steel.
Quartz is transparent to translucent, semi-translucent to opaque,
doubly refractive, and does not melt before the ordinary blow-pipe, but
may be melted with the oxyhydrogen blow-pipe. It also melts with soda
to a clear glass, and is soluble in fluohydric acid.