Quartz, the most abundant of all minerals, existing as a constituent of many rocks, as the granitic and the micaceous and silicious slates, composing of itself the rock known as quartz-ite or quartz rock and some of the sandstones and pure sand, forming the chief portion of most mineral veins, and found interspersed in crystals and crystalline fragments throughout many rocks, and especially in their fissures and cavities. In composition it is silica, and when uncontaminated with any foreign intermixture it appears in clear transparent crystals like glass or ice. The presence of a little oxide of manganese gives these a violet tinge, and they are then known as amethyst. Other impurities which variously affect the appearance and properties of quartz, even in the small quantities in which they enter into its composition, are oxides of iron, aluminum, nickel, and other metals. Through all its varieties quartz is distinguished by the same chemical properties and degrees of hardness. This, which enables the mineral to scratch glass and to give fire when struck with steel, is represented by 7 of the scale of hardness. Its specific gravity is 2.5 to 2.8. Its lustre is vitreous, its colors various according to the impurities present, and its fracture conchoidal.

It is fusible only at the intense heat of the oxyhy-drogen blowpipe, and of the furnaces invented by Saint-Claire Deville; but it is readily fluxed with soda or lime. The quartz glass obtained by Deville, amounting to 30 grammes, possessed a density of only 2.2, or about one seventh less than that of the crystallized quartz from which it was melted. The colorless. transparent crystals impress circular polarization upon a ray of plane-polarized light. They exhibit double refraction when the object is observed through two faces which are not parallel to each other. Milk-white varieties often give a phosphorescent light when rubbed together in the dark. The primary form of the crystal, which is very rarely seen, is a rhomboid. The common form is a hexagonal prism terminated by hexagonal pyramids. The crystals occur in groups of great beauty and of all sizes up to single crystals of several hundred pounds' weight. In the museum of the university at Naples is a group weighing nearly half a ton. In Milan is a crystal 3 1/4 ft. long and 5 1/2 ft. in circumference, estimated to weigh 870 lbs. A crystal in the museum of natural history in Paris is 3 ft. in diameter and the same in length, and weighs 800 lbs.

Occasionally immense quantities of crystals are found collected in cavities in the rocks and in caves, loose and incrusting the walls. Such a collection, discovered at Zinken more than a century ago, produced 1,000 cwt. of rock crystal, which at that period, when the article was more highly valued than now, brought $300,000. In the United States some rich deposits have been met with in the Ellenville lead mine, Ulster co., N. Y., and in some of the southern gold mines; and large groups of fine crystals have been found in Moose mountain, N. H., and in Waterbury, Vt. Little Falls on the Mohawk in New York is a famous locality for the purest transparent crystals of complete forms, and they are met with in other places also in the same region, occurring in the cavities of the calciferous sand rock, which overlies the Potsdam sandstone. Trenton Falls also furnishes perfect transparent crystals, which are sometimes 5 in. long and contain drops of water. These are occasionally recognized in quartz crystals of various localities. In St. Lawrence and Jefferson cos., N. Y., in the deposits of iron ore, quartz crystals are found of dodecahedral forms. In Orange co., 4 m. E. of Warwick, they occur in the primary form.

Many of the varieties of quartz are known by other names, under which they have been particularly described in this work. (See Agate, Amethyst, Carnelian, Cats' Eye, Chalcedony, Flint, Geode, and Jasper.) - Clear crystalline quartz, called rock crystal, was in former times esteemed for ornamental objects. It was cut into vases, cups, lustres, etc., many of which are still preserved as curiosities. In the museum of the Louvre are great numbers of them, some belonging to the times of the ancient Greeks and Romans, but more generally of the period of the middle ages. The perfection to which the manufacture of glass and pastes has been brought and the cheapness of these materials have almost completely taken away the value of rock crystal, which requires a great amount of labor in its cutting and polishing, and after all is not really superior to the artificial products. But some use is still made of it, as for buttons, seals, breast pins, etc. It is procured from Madagascar, Switzerland, and Brazil. Very transparent specimens from the latter country are made into spectacle lenses called " Brazilian pebbles." They are superior to glass on account of their greater hardness. In Switzerland quartz veins which occasionally yield rich cavities of crystals are regularly mined for this product.

From Madagascar large clear masses are received, which sell for from $1 to $10 a pound. When cut and set by the jewellers, the stone is commonly sold as white topaz, and sometimes as "California diamonds." Pure quartz is largely employed in the manufacture of glass, and is commonly obtained for this purpose in the form of sand; but metamorphic quartz rock of a granular structure and crumbly consistency is also used. (See Glass.) Varieties of quartz of a cellular texture and great tenacity are used for millstones, the roughness and hardness of their surface and sharpness of the edges of the cells giving them a powerful grinding capacity combined with durability. (See Buhrstone.) Quartz veins with few exceptions form the gangues in which gold is found in situ, and it is probable that most of the gold which is obtained from alluvial and drift deposits came originally from the quartz veins. These gold-bearing quartz veins intersect various metamorphic rocks, such as chloritic, talcose, and argillitic schist, hornblende schist, gneiss, porphyry, and sometimes granite. (See Gold).