RUBY, the most valued of all gem-stones, a red transparent variety of corundum, or crystallized alumina (Lat. rubeus, red). It is sometimes termed "oriental ruby" to distinguish it from the spinel ruby (q.v.), which is a stone of inferior hardness, density and value.
The ruby crystallizes in the rhombohedral system (see CO RUNDUM) ; the crystals have no true cleavage, but tend to break along certain gliding planes. The colour varies from deep cochi neal to pale rose-red, in some cases, with a tinge of purple, the most valued tint being that called by experts pigeon's-blood col our. The oriental ruby is a mineral of very limited distribution. It most famous localities are in Upper Burma, principally in the neighbourhood of Mogok, 90 m. N.N.E. of Mandalay. It occurs in bands of a crystalline limestone, associated with granitic and gneissose rocks, some of which are highly basic; the limestone also contains spinel, garnet, graphite, wollastonite, scapolite, fel spar, 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, etc., may yield margarite,
vermiculite, chlorite and other hydrous silicates.
Rubies have been produced artificially (see GEMS, ARTIFICIAL) with much success. It was once the practice to make "recon structed rubies" by fusing together small fragments of the natu ral stone; but this process has given way to Prof. 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 corun dum, but may generally be distinguished by microscopic bubbles and striae. The manufacture of synthetic rubies is carried out commercially.