Diamond

Diamond (named from the Greek adamas, unsubduable), the hardest of precious stones, being 10 in von Mohs' scale. It consists of pure carbon, burning in strong heat or in pure oxygen into carbonic acid gas. It is insoluble in acids. From graphite (q.v.) it differs in crystalline system and in transparency. The forms in which it occurs, the octahedron, rhombic dodecahedron, triakisoctahedron and hexakisoctahedron, as well as the rarer cube, all belong to the Cubic system. Some of the crystals are nearly spherical, having curved faces and edges. The mineral is rather brittle, cleaving readily parallel to the octahedral faces. Its specific gravity is 3.52. Diamond when at its purest is colourless, transparent, adamantine in lustre, with the high refractive index 2.42; but it also occurs grey, brown, blue, red, yellow, green, or black. Diamonds were formerly only obtained from the Deccan, where they occurred in recent derivative deposits, no original matrix being known. Since the beginning of the last century they have been obtained from Minas Geraes, Matto Grosso, and Bahia, in Brazil, in itacolumite, a quartzose mica-slate or metamorphosed sandstone, and in soil derived from it. Since 1867 diamonds have been largely obtained from Griqualand West and the Orange Free State, in South Africa, where the town of Kimberley has arisen, forty million pounds' worth having been exported since the first discovery. They were found at first in alluvial shingle; but subsequently in "pipes" of a shaley decomposed diabase, or closely related rock of igneous origin. The mode of origin of the mineral is by no means clear, and no satisfactory demonstration has as yet been given of its having, as alleged, been prepared artificially. Diamonds are cut and polished by means of a wheel armed with their own powder, the industry being carried on in Amsterdam and London. Three forms are cut, the brilliant, the rose, and the table. The brilliant consists of a double pyramid, the upper and more truncated half having a table or horizontal eight-sided facet, surrounded by a bezil or series of smaller facets, whilst the lower and more pyramidal half extending from the girdle, or common base of the pyramids, to which the setting is attached, is called the collet side, and terminates in a minute flat facet, the collet. The rose is flat on one side, with triangular facets, of which the central ones meet in a point on the other. The table, adopted only for thin stones, has two parallel, wide, tabular facets with rows of smaller ones between them. The weight of diamonds is reckoned by the carat, which is a little more than 205 milligrams. The price of brilliants of first quality as to colour or "water," well cut, but not of exceptional size, is reckoned at from £8 to £10 for the first carat, increasing as the square of the weight in carats, e.q. two carats. £32 to £40, three carats £72 to £90. Among the most celebrated large diamonds are the Orloff, in the Russian sceptre, 194-3/4 carats, and the Koh-i-Noor, 106-1/10 carats, since the annexation of the Punjaub in 1849 the property of her Majesty. Besides its value as a gem, diamond is used by glaziers for cutting glass, and when imperfect in form and brilliancy, under the name of bort, for polishing. A brownish black porous impure variety, with a density less than 3.42, is found near Bahia in large masses, and, under the name carbonado, is now largely used in boring wells, trials for mines, etc., several pieces of it being set round the margin of a revolving steel tube or "crown."