Spinel (Fr. spinelle), a mineral, sometimes ranked among the precious stones, occurring in regular octahedrons and dodecahedrons, variously modified; hardness, 8; specific gravity, 3.5 to 4.9. The color is commonly some shade of red, but is sometimes blue, green, yellow, brown, black, and rarely almost white. When pure, it is a compound of magnesia 28, alumina 72; but the magnesia is often replaced to some extent by one or more of the protoxides of iron, zinc, or manganese, or by lime, and the alumina also by peroxide of iron; hence the numerous varieties of the species. These are denominated according to their colors, and some among them are often supposed to belong to other species. The black varieties are called pleonaste; the scarlet, spinel ruby; the rose red, balas ruby; the yellow, or orange red, rubicelle; the violet, almandine ruby; and the green, ceylonite. The goutte de sang of the jewellers is of blood-red or cochineal color. The mineral is infusible before the blowpipe alone, and is not attacked by acids. The most valuable spinels are found in Ceylon, Siam, and other eastern countries, in the form of rolled pebbles in river beds.

They are also found in New Jersey, New York, and central Massachusetts. Perfect specimens fit for jewelry are rare; if of more than four carats, they are sometimes rated as worth half as much as diamonds of equal size. The red varieties are said to be sold for true rubies, from which they are with difficulty distinguished; and many of the others are often confounded with other precious stones of similar hardness and specific gravity. The optical properties alone may decide without analysis between the colorless spinel and the limpid topaz of Siberia. Dufre-noy was obliged to apply the test of polarization of light to a white cut spinel from India, which was supposed to be either a diamond or a white emerald. He describes one of a clear crimson with a violet tint, weighing 1,129 grains, of great beauty, valued at 100,000 to 110,000 francs.