The Red Corundum

Although we place the ruby fourth among the precious stones, so few are the superior rubies in commerce, or that the world sees, that when a perfect ruby of the weight of ten or more carats enters the market, it brings a price three times as great as does a diamond of the same weight.

The natives of India indiscriminately apply the name "ruby" to all coloured precious stones, and it is the habit of American dealers in precious stones to be almost as general in calling various red gems rubies, although they do distinguish by calling the corundum ruby "Oriental ruby." This being a book for every-one, other red stones commonly or even occasionally appearing in the jewelry trade and called by merchants rubies will be comprehended and described in this chapter, leading with the corundum reality, which is beyond compare.

Corundum crystallises in the hexagonal system in six-sided prisms and pyramids, the crystals frequently being rough and rounded; hardness 9; brittle; specific gravity 3.9 and upwards to 4.16; lustre adamantine to vitreous; sometimes the lustre is pearly on the basal plane; and occasionally there is exhibited a bright, opalescent, six-rayed star in the direction of the vertical axis. The colour range is almost unlimited, blue corundum being sapphire. The strongly coloured varieties are pleochroic. Corundum is sometimes phosphorescent, with a rich red colour. The red-coloured corundum or ruby varies from a rose to a deep carmine, the desideratum being a "pigeon's blood" red, and the same crystal will sometimes reveal different colour. Like its brother in the noble corundum family, the ruby is a peer of the realm of precious stones, and second only to the throne of the sovereign diamond.

In chemistry, corundum is pure alumina, the oxide of the metal aluminum, composed of 53.2 per cent. of the metal and 46.8 per cent. of oxygen. Natural corundum is probably never chemically pure; the inclusions of foreign elements, sometimes but the merest traces, impart the colour that makes the gem. When foreign matter is present in large proportion corundum is impossible for gem purposes, although of great value industrially; inferior translucent specimens serve for pivot supports of watches and other delicate machines and the opaque as an abrasive; thus common corundum is used for cutting and polishing gem minerals lower in the scale of hardness than the diamond, a variety of it being the common compact black emery powder used for sharpening and polishing in mechanical and domestic uses, and familiar to everyone.

A chemical analysis of a fine specimen of an "Oriental ruby," of the approved rich deep red hue was as follows: alumina, 97.32; iron oxide, 1.09; silica, 1.21; in all, 99.62. The extent to which crystallography goes and its fine, yet plain, distinctions, in determining gem minerals, are illustrated by the marked crystallographic differences between the ruby and the sapphire, which differ but slightly in chemical composition, having the same constituents but different proportions; thus one typical sapphire analysed entire exhibited alumina, 97.51; iron oxide, 1.89; and silica, 0.80; in all, 100.20. The forms of corundum generally occur in two different habits represented by the ruby and the sapphire; in the former the prism predominates and in the latter the hexagonal pyramid.