It is only during the past century that mineralogists make a distinction between the minerals spinel and corundum.

The composition of the spinel was discovered towards the end of the last century, and was found to be about seventy per cent. alumina, twenty-five per cent. magnesia, and small parts of oxide of chrome, silica, and protoxide of iron. Up to that time, red spinels had always been confounded with rubies, and many celebrated so-called rubies have been shown to be spinels by modern mineralogists.

This beautiful mineral is found in many colors, from pink to rose-red, carmine, cochineal, blood-red, hyacinth, pale to dark blue, violet and indigo blue, grass-green to blackish green, and sometimes colorless. There is also a black variety called pleonaste or ceylonite. Spinels crystallize in octahedrons and their modifications, the fracture is conchoidal, specific gravity 3.5 to 3.6, and hardness No. 8 in Moh’s scale; only the diamond, corundum and chrysoberyl will scratch the spinel.

Its refraction is single, the lustre highly vitreous, and it does not easily acquire electricity.

Acids do not attack the spinel, nor has the blow-pipe any effect on this mineral, except to change the red to a brownish or colorless state, but the original color returns when the stone cools.

Flawed or imperfect stones are liable to crack or split if heated too much. With borax or salt of phosphorus the spinel melts into a colorless or green-tinted glass.

Spinels are found in clay and in the sands of rivers, in East India, Hindustan, the province of Mysore, Farther India, Pegu, Ceylon, North America, Sweden, Bohemia, and Australia.

The red spinel, and especially those tints which approach the red corundum or true ruby in color, are the most valuable, and are known as ruby spinels.

Very fine specimens of ruby spinels of one carat and larger are quite rare and command good prices.

Rose-colored spinels are known as balas-rubies, pale-blue spinels as sapphirines, and the hyacinth-red, yellowish-red, and orange-yellow spinels are called rubicelles.

All these different-colored spinels, if pure and of great brilliancy, are valuable as gem stones, being only surpassed in hardness and brilliancy by the diamond and corundums.

The white spinel, which is seldom found, is sometimes confounded with the diamond, having the same specific gravity and single refraction, but as it lacks the fire and is easily scratched by the diamond, the danger of mistaking one for the other is slight. Burnt amethyst, which often resembles the spinel, is lighter and softer, while burnt topaz, although it is identical with the spinel in hardness, is somewhat lighter and possesses remarkable electric powers, becoming electric by either rubbing, heating, or pressure, and retaining electricity for upwards of twenty-four hours.

The zircon is easily distinguished from the spinel because of its much greater specific gravity. It is also doubly refractive and softer.

Garnets are softer, lack the play of color and brilliancy, and fuse easily into a light-brown or black glass.