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.

Quartz is composed of pure silica

Some of the impure varieties contain oxide of iron, carbonate of lime, clay, and other minerals.