QUARTZ, in mineralogy, a species of the Flint genus, which is divided into five sub-species, viz, the AMETHYST, which see ; the rock-crytal ; milk-quartz ; com mon-quartz ; and prase. The rock-crys tal is white, passing to brown through all the intermediate shades. It occurs rarely massive• often in rolled pieces, and often in crystals of different forms. Externally, the crystals are generally splendent, the rolled pieces are only glistening ; inter nally, they are splendent and vitreous. It is harder than glass, and gives vivid sparks when struck against steel. It is brittle, and easily frangible. Specific gravity 2.65 when pure, but when deeply coloured by metallic oxides, it is consi derably more. If two of the crystals are rubbed together they afford a phospho rescent light, and exhale a peculiar odour. Ily exposure to the blow-pipe, this crys tal undergoes no change, except the loss of colour. It remains unaltered even when exposed to a stream of oxygen gas. It is composed of Silica 93 0 Alumina 6 0 Lime 1 100 It is found in abundance in the Alps, also in Hungary, Saxony, and in many parts of the British islands. It is used as an article of jewelry, and is very much prized, particularly the wine and orange yellow.
Milk-quartz is sometimes of white co lour, but more frequently of a rose red, passing through all the degrees of inten sity to a flesh red. It occurs massive internally shining : sometimes passes to splendent, and is vitreous, inclining a lit tle to resinous. Hard, but yielding to the file ; easily frangible, and not very heavy : it is imagined to be composed of silica and oxide of manganese. It is found in beds, but never in veins, in primitive mountains, in Germany, Sweden, Green land, Siberia, and also in Coll, one of the Hebrides. It is employed in ornamental
works, takes a good polish, and when the colour is good the ornaments made of it are very beautiful. It loses its colour by keeping in a warm place.
Common-quartz is commonly of a white or grey colour, though many specimens are brown, yellow, red, &c, It is found massive, disseminated, in blunt edged pieces, in roundish grains, and rolled pieces. It occurs also in crystals of dif ferent kinds. Externally, the lustre of the true crystals varies from splendent to glistening : internally, it is shining and vitreous. Fragments angular, and sharp. edged, massive. Occurs commonly un separated, but often in prismatic distinct concretions, which are straight, trans versely streaked. It is hard, brittle, easily frangible. Specific gravity about 2.6. It is infusible, without addition, before the blow-pipe ; but when exposed to a stream of oxygen gas, it melts into a white por cellanous ball. It occurs abundantly in the mineral kingdom, and found forming whole rocks, also in beds and veins, and is a constituent part of granite, gneiss, mica, slate, &c. It is employed in place of sand in the manufactory of glass, also in the preparation of smalt, and as an in gredient in porcelain and different kinds of earthenware.
Prase is of a leek-green colour, of va rious degrees of intensity. It occurs ge nerally massive, seldom crystallized; it is hard, difficultly frangible, not very heavy. It is found in Saxony, in Finland, and Siberia ; and is sometimes cut and polished for ornamental purposes.