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.