First Class Precious Stones: Quartz- Physical Characteristics and Location

Quartz, which is called also rock crystal, is one of the substances most frequently occurring upon the surface of the earth, and probably also in its interior. "It forms," says an English writer, "about one-third of the mass of those immovable hills whose summits pierce the clouds, and nearly the whole of the mobile soil of the trackless desert rolling with the wind like the waves of the sea."

Crystals of quartz do not often occur of dimensions sufficiently great to attract the eyes of the common observer. Some magnificent specimens, however, are found in the ancient formations, which, as we have mentioned, are formed principally of silica: here indeed we would naturally look for them; but that which would seem much less probable, and yet actually occurs, is the presence of magnificent crystals of quartz, of an absolute purity, in the midst of rocks nearly exempt from all trace of silica; in Carrara marble, for example, and in certain gypseous formations in the south of France.

Quartz is formed by the union of two bodies; the one, silicon, is a substance analogous to carbon; the other, oxygen, is a gas, and one of the principal constituents of atmospheric air.

The primitive form of quartz is the rhombohedron, but the primitive crystals are extremely rare. The most common form is the regular hexagonal prism terminated with six-sided pyramids (Fig. 68).

It is rare for the terminal pyramids to have all the faces equal. Ordinarily, on the contrary, three of these faces are developed at the expense of the other three, and thus we have the crystal represented by Fig. 69.

In other cases the crystals are not terminated by pyramids, but by ridges, as in Fig. 70. In this case the form of the crystals is greatly altered, and the regularity, to a certain extent, disappears.

If in the regular crystal (Fig. 68) we suppose the prismatic part to be diminished little by little without the form being otherwise changed, then, when the prismatic portion is quite removed, and the pyramids are applied base to base, the crystal represented by Fig. 71 is obtained. It is a dodecahedron, all of whose faces are equal, and bounded by isosceles triangles.

Crystals of this form occur in the midst of the gypsum which accompanies the ophites of the Pyrenees. Numerous examples of perfect purity occur also in the gypsum beds of Provence, which belong to the trias formation.

Crystals do not ordinarily attain large dimensions. For the greater number of minerals, crystals of 2 inches are almost gigantic: few indeed exceed 4 inches in height. Quartz, however, forms an exception to this rule. Specimens are brought from Madagascar more than 12 inches in length, and remarkably pure and transparent, notwithstanding their great size. The rock crystal of this island is used for the object-glasses of astronomical telescopes. Magnificent crystals have also been found in the Alps; one of these Alpine crystals, taken in Italy by the French, was borne in triumph to Paris in 1797. There is a beautiful specimen in the Museum of Natural History at Paris which measures 3 feet every way, and weighs nearly 800 pounds. It was found at Fischbach in the Valais.

At the French Exhibition of 1866, in the sections of Japan and of Brazil, there were some wonderful crystals. One crystal brought from Brazil weighs 212 pounds, is 2 1/2 feet high, and 1 foot in diameter, and is a perfect six-sided prism.