Spinel and Other Rubies

Of the other stones than corundum called "ruby," the only important ones are the varieties of spinel, which chemically is closely allied to corundum so that the red varieties of spinel might be regarded as cousins-german to the real ruby. The "Cape ruby"-so called in the jewelry trade-is pyrope garnet from the diamond-bearing rock of South Africa, and is described in its proper place-the chapter on the garnet. Stones sometimes substituted for the ruby by dealers, or mistakenly called rubies, are red tourmaline, or rubellite, called "Siberian ruby"; rose topaz, called "Brazilian ruby"; and hyacinth or jacinth, which is zircon, and is described in the chapter on "Semi-Precious Stones Occasionally Used." Spinel has perhaps a wider range of colour than almost any other mineral, but it will be considered here chiefly with regard to the red varieties approximating the colour of the ruby. Spinel is practically a magnesium aluminate, consisting of alumina, 71.8%, and magnesia, 28.2%. The chief red shades are: deep red, Siam ruby and spinel ruby; rose red, balas ruby; yellow or orange red, rubicelle; violet red, almandine ruby. The native name in India for spinel is "pomegranate." A slight knowledge of mineralogy should suffice to distinguish the corundum ruby from its spinel distant relative, for the latter is less hard and of lower specific gravity, and different in crystallisation. Spinel is of about the hardness of topaz, or 8 in the Mohs scale, and its specific gravity is about 3.6. It crystallises in the isometric system and usually appears in the form of octahedrons. It is singly refracting, corundum doubly. Spinel is infusible before the blowpipe, but heating it will cause it to undergo several changes of colour, ultimately returning to its original hue, so that it might be termed the chameleon of gem minerals. Without any design to substitute spinel for corundum rubies, spinel has its own deserved value, and its beauty and intrinsic worth deserve for it an inclusion in the company of the high-class gems. It is interesting to note that spinel ruby is not only the relative of the patrician corundum ruby, but the poor relation dwells together with its wealthy relative in nature. Both rubies are found associated in the gem gravels of Ceylon, Siam, Australia, and Brazil, as well as in the crystalline limestone of upper Burma. Spinel rubies are found in quantity in Balakschan, Afghanistan, near the River Oxus; the name "Balas ruby" is probably derived from Beloochistan, otherwise Balakschan.