RUBY (Lat. rubeus, red), the most valued of all gem-stones, a red transparent variety of corundum, or crystallized alumina. It is sometimes termed “oriental ruby ” to distinguish it from the spinel ruby, which is a stone of inferior hardness, density and value (see Spinel). When the word ruby is used without any qualifying prefix, it is always the true or so-called oriental stone that is meant in modern nomenclature. Ancient writers, relying chiefly on colour, classed together under a common name several brilliant red stones, such as the ruby, spinel and garnet: thus the ἄνθραξ of Theophrastus and the Carbunculus of Pliny were names which seem to have been applied to several distinct minerals. Although the word ruby is used in the English translation of the Old Testament it is improbable that the true ruby was known to the ancient Hebrews.
The ruby crystallizes in the hexagonal system (see Corundum). The crystals have no true cleavage, but tend to break along certain gliding planes. The colour of ruby varies from deep cochineal to pale rose-red, in some cases with a tinge of purple, the most valued tint being that called by experts pigeon's-blood colour. On exposure to a high temperature, the ruby becomes green, but regains its original colour on cooling. The red colour of ruby may be due to chromium. When a ruby of the most esteemed tint is examined with the dichroscope, one image is generally seen to be Carmine and the other aurora-red, the red colour inclining to orange. This test serves to distinguish the true ruby from spinel and from garnet, since these minerals, being cubic, are not dichroic. Another means of distinction is afforded by the specific gravity of ruby (about 4), which is higher than that of spinel and garnet, whilst the superior hardness of the ruby (about 9) furnishes yet another test. The high refractivity of ruby is also characteristic, the mean ordinary index being 1.77 and the extraordinary 1.76. When cut and polished the ruby is therefore a brilliant stone, but having weak dispersive power it lacks fire. Subjected to radiant discharge in a Crookes tube, the ruby, like other forms of corundum, phosphoresces with a vivid red glow.
The oriental ruby is a mineral of very limited distribution. Its most famous localities are in Upper Burma, but until the British annexation of the country in 1886 the mines were so jealously guarded that little was known as to the conditions under which the mineral occurred. Soon after the annexation, the ruby districts were officially visited, and reported on, by Mr C. Barrington Brown, and specimens from the mines were exhaustively studied by Professor J. W. Iudd. The principal district is situated in the neighbourhood of Mogok, 90 m. N.N.E. of Mandalay. The ruby occurs in bands of a crystalline limestone, associated with granitic and gneissose rocks, some of which are highly basic; and it is from the anorthite, or lime-felspar, and the associated minerals in the pyroxene-gneisses, that the corundum, spinel and calcite, may, according to Judd, have been derived. Probably the felspar is first altered to scapolite, and this on decomposition would yield calcium carbonate and hydrous aluminium silicates, from which the anhydrous alumina might ultimately be separated. The limestone contains (in addition to the ruby) spinel, garnet, graphite, wollastonite, scapolite, felspar, mica, pyrrhotite and other minerals. The ruby, like other kinds of corundum, suffers alteration under certain conditions, and passes by hydration into gibbsite and diaspore, which by further alteration and union with silica, &c., may yield margarite, vermiculite, chlorite and other hydrous silicates.
The Burmese rubies are not generally worked in the limestone matrix, but are mostly found loose in detrital matter, which is clayey and sandy in character and yellowish-brown in colour, and is known locally as “ byon.” Some of the deposits occur in limestone caverns, where they may, like cave-earth, represent the insoluble residue of the limestone. Workings in the cave-deposits are called “ loodwins ” (crooked mines). In the alluvium of the valleys, the ruby-pits are known as “ twinlones ” (round pits), whilst workings in the ruby-earth on the hillsides are termed “ hmyaudwins ” (water mines). The byon contains, with the ruby, other coloured corundums and spinels. Burmese rubies are found also in crystalline limestone in the hills near Sagyin, about 20 m. N. of Mandalay, and it is of mineralogical interest to note that the limestone here contains chondrodite.
Rubies are found in Siam, at several localities in the provinces of Chantabun and Krat; and Professor H. Louis has described their occurrence at Moung Klung in this region. The rubies are found with sapphires and spinels, in gravels, resting in some cases on basic igneous rocks. The Siam rubies are generally of dark colour, often inclining to a deep reddish brown. Rubies occur, with sapphires and other minerals, in the gem-gravels of Ceylon, but are not usually of such good colour as the Burmese stones. A cloudy variety, which, when cut with a convex surface, exhibits a luminous star, is known as star-ruby (see Asterias). In peninsular India rubies are rarely found, though they have been reported from the corundum deposits of Madras and Mysore. The ruby is known, however, to occur in a micaceous limestone at Jagdalak, near Kabul in Afghanistan.
Rubies, generally of pale colour, are found with the sapphires of Montana, especially at Yogo Gulch near Utica. In the corundum deposits of N. Carolina ruby is occasionally met with, especially at Cowee Creek, Macon county, where it occurs in crystals of tabular, rhombohedral and prismatic habit. These crystals, sometimes of fine colour, are found in gravels resting on a soft rock called saprolite, which results from the weathering of certain basic igneous rocks; and it is notable that the ruby crystals are associated with the variety of garnet termed rhodolite, as described by Professor Judd and W. E. Hidden. Australia has occasionally yielded rubies, but mostly of small size and inferior quality. In New South Wales and in Victoria they have been found in drift gravels, and a magenta-coloured turbid variety from Victoria has been described under the name of barklyite.
Rubies have been produced artificially with much success. At one time it was the practice to fuse together small fragments of the natural stone; and gems cut from such material were known as reconstructed rubies. This process has given way to Professor A. Verneuil's method of forming artificial ruby from purified ammonia-alum with a certain proportion of chrome-alum. The finely powdered material is caused to fall periodically into an oxyhydrogen flame, the heat of which decomposes the alum, and the alumina thus set free forms liquid drops which collect and solidify as a pear-shaped mass. When of the characteristic pigeon's-blood colour, the synthetical ruby contains about 2.5% of chromic oxide. The manufactured ruby possesses the physical characters of corundum, but may generally be distinguished by microscopic bubbles and striae. The manufacture is carried out commercially. (For other processes, see Gem, Artificial.)
It should be noted that several minerals known popularly as rubies have no relation to the true red corundum. Thus, “ Cape rubies ” from the South African diamond mines, “ Australian rubies ” from South Australia, and “ Arizona rubies ” are merely fine garnets; “ Siberian ruby ” is red tourmaline (see Rubellite), and “ Balas ruby ” is spinel (q.v.). Ruby silver is a name applied to light red silver ore, or proustite; ruby copper is merely cuprite, in brilliant crystals; and ruby-blende is a clear red variety of zinc sulphide.
Bibliography.—For the Burma ruby, see “ The Rubies of Burma and Associated Minerals: their mode of occurrence, origin and metamorphoses," by C. Barrington Brown and Professor J. W. Judd, Phil. Trans., 1897, 187, p. 151. For the ruby of Siam, see " The Ruby and Sapphire Deposits of Moung Klung, Siam, " by H. Louis, Mineralog. Mag., 1894, 10, p. 267. For synthetical ruby, see G. F. Herbert Smith, Mineralog. Mag., 1908, 15, p. 153; and J. Boyer, La Synthèse des pierres précieuses (Paris, 1909). (F. W. R.*)