Quartz, a mineral composed of silica, . It is met with chemically pure, but not infrequently contains variable proportions of ferric oxide, manganese oxides, alumina, magnesia, lime, organic matter, &c. Very often it shows inclusions, microscopic or macroscopic as the case may be, of various minerals and fluid cavities. It occurs both in crystals and massive, the more common crystals being hexagonal prisms terminated by hexagonal pyramids. Double hexagonal pyramids are also not uncommon. It scratches glass easily, and becomes positively electrical by friction—two pieces rubbed together giving light in the dark. Quartz when pure is colourless, but, owing to the presence of foreign substances, many coloured varieties are known. Three types of quartz are recognised: (1) Rock-crystal, (2) Common Quartz, and (3) Compact Quartz.
Rock-crystal.—Under this head are included the varieties which are more or less transparent and assume well-marked crystalline forms. The water-clear crystals are known as Rock-crystal. The crystals are sometimes slender, crossing and penetrating each other in exquisite groups. They frequently enclose other substances, which are beautifully seen through the transparent rock-crystal, as slender hair-like or needle-like crystals of hornblende, asbestos, oxide of iron, rutile or oxide of titanium, oxide of manganese, &c., and such specimens are known by various fanciful names, as Thetis' Hair-stone, Venus' Hair-stone, Venus' Pencils, Cupid's Net, Cupid's Arrows, &c.; and sometimes the enclosed substances are small spangles of iron-glance, or crystals of iron pyrites, or native silver in fern-like leaves, or spangles of gold. Fluid inclusions are also not uncommon, as in the quartz of Poretta. Very large crystals of perfectly pure rock-crystal are sometimes found.
One from the Alps, which was among the treasures carried from Italy by the French in 1797, is 3 feet in length, about 1½ feet in diameter, and weighs 7 cwt. Similar giant crystals are obtained in Madagascar. Rock-crystal was prized by the ancients, and was used by them, as it still is, for vases, cups, seals, &c. An important modern use of it is for lenses of spectacles, &c., its hardness rendering it much less liable to be scratched than glass. Lenses of rock-crystal are often called Pebble lenses. Rock-crystal is best developed in the crevices and cavities of crystalline schistose and granitoid rocks, such as those of Tyrol and the Alps, where it is associated with felspar, titanite, rutile, mica, chlorite, and other crystallised minerals. Smoky Quartz, smoky-brown; Cairngorm, smoky-yellow; and Morion, black, are varieties the colours of which have been variously attributed to the presence of small quantities of oxide of iron or manganese, or titanic acid or organic substance. Yellow and pellucid varieties of rock-crystal are known as False Topaz. Amethyst (q.v.) or Amethystine Quartz is purple or violet; the darker shades are often highly prized; sometimes speckled varieties of this beautiful mineral occur. When subjected to heat amethyst loses its violet colour and becomes yellow. The Gold Topaz and Citriu of jewellers are in most cases amethysts which have been treated in this way.
Common Quartz includes non-transparent varieties, some of which, however, are more or less translucent. They are either white, colourless, or coloured, the tints being generally pale, but many show intense shades of red, green, yellow, brown, &c. Sometimes they assume crystalline forms, at other times they have a granular or massive structure. The following are varieties: Milk-quartz, milk-white and slightly opalescent (Greenland, &c.); Greasy Quartz, like milk-quartz, but with a greasy lustre; Rose-quartz, rose-red, sometimes crimson, the colour fading on exposure (Bodenmais); Siderite or Sapphire-quartz, indigo or blue in colour, from the presence of asbestos-like fibres of blue crocidolite (Golling in Salzburg); Ferruginous Quartz, coloured red with ferric oxide or brown with hydrous ferric oxide; Prase, leek and other shades of green, which are due to needles of actinolite (Saxony, &c.); Star-quartz, containing within the crystal whitish or coloured eccentric radiations (Bohemia); Fibrous Quartz, brown, with a silky lustre, pseudomorphous after crocidolite (Cape of Good Hope); Quartz-pisolite is an oolitic aggregate of quartz granules (Sicily, Egypt); Floatstone, porous, cavernous, which floats in water until the air in its numerous cavities becomes displaced (Cornwall, &c.); Cat's-eye, yellow or greenish, exhibits opalescence, but without prismatic colours, an effect due to fibres of asbestos or minute tubes arranged in parallel directions (Ceylon, Harz, Fichtelgebirge, &c.).
Compact Quartz embraces compact and also finely granular aggregates, which are only translucent on their edges. Varieties are Hornstone, gray, brown, yellow, or red, common in many countries; Chrysoprase, a hornstone coloured green with nickel (Silesia); Aventurine (q.v.), a somewhat granular hornstone, spangled with scales of mica or goethite (Altai Mountains); Jasper (q.v.), brown, yellow, red, very impure, with a considerable percentage of iron.
Another group of minerals which are included in the quartz family are the chaledonics. These are mixtures of crystalline and amorphous silica. Chaledony (q.v.) itself is colourless or pale gray, or pale bluish or yellowish, translucent to semi-opaque, and occurs either in irregular layers or in mammillary, botryoidal, or stalactitic forms. The following are coloured chaledonies: Carnelian, clear red and sometimes yellowish; Plasma, green; Heliotrope or Bloodstone (q.v.), green with red spots. Cacholong is a dull milk-white and sometimes porous chaledony, but is regarded by some mineralogists as a variety of opal. Mocha-stone or Moss-agate is a colourless chaledony, containing brown moss-like dendritic inclusions. Agate (q.v.) consists of alternate layers of chaledony and other varieties of crystalline and amorphous silica. Onyx is composed of even layers of black or brown and white chaledony; while in Sardonyx, which has a similar structure, the alternate layers are red and white. For amorphous forms of silica which contain variable quantities of water, see OPAL.
Quartz is the most abundant and widely diffused of all rock-forming minerals. It is an important and characteristic ingredient of acid igneous rocks, such as granite, quartz-porphyry, &c., and forms one of the principal constituents of gneiss. In many other igneous and schistose rocks it is met with as a more or less prominent constituent. It occurs also abundantly as a secondary mineral or alteration-product in igneous and schistose rocks generally—sometimes irregularly diffused, at other times occupying cracks, crevices, cavities, &c. Many rocks, especially Archæan and Palæozoic rocks, are traversed by veins of quartz, which vary in breadth from mere lines up to many feet. In some places such veins are more or less impregnated with gold—indeed quartz-veins may be looked upon as the chief repositories of the precious metal (see GOLD).
Quartz, in crystals, is also one of the most common minerals met with lining the walls of metalliferous lodes. Amongst derivative rocks it plays an important part—conglomerates and sandstones being composed as a rule chiefly of siliceous materials. Chalcedony as a rock-constituent is a product of alteration, and is met with commonly in the vesicles and pores of many igneous rocks, or as irregular aggregates diffused through their ground-mass. It is also a common vein-mineral—the coloured chaledonies especially occurring in this form. Agates occur in veins, and especially in the amygdaloidal cavities of igneous rocks.