Composition—Essentially alumina and magnesia, but the latter is often replaced by protoxide of iron, and by lime. MgO, A1,03 if pure, containing magnesia, 28, alumina 72 per cent., but the black specimens contain as much as 20 per cent, of protoxide of iron.
This mineral covers a wide range as regards colour, more in fact than any other precious stone. It is found in many shades of red, blue, green, yellow, brown, black, and sometimes nearly white.
The most marked of these colours give to this mineral its distinctive names, and its varieties, on account of this difference in colour alone, are known as follows :—The deep red as spinel ruby, the rose-red as balas ruby, the violet as almandiue ruby, the orange-red as rubicelle, and the black as pleonaste.
The very dark-coloured specimens are of no importance as gem-stones; it is only the transparent varieties that are of value, and these are often spoken of as precious spinels. The deep red stones often rival the ruby in colour, but they are inferior in "fire" to that gem, owing to the small refractive and dispersive power possessed by the spinel; and to its absence of pleiochroism may be attributed its deficient brilliancy. Crystallizing in the cubical system, it cannot of course be dichroic; but in spite of these defects, large quantities are used for jewellery, were originally considered identical with the Oriental ruby, and were credited with possessing the same supernatural powers as that gem. Even now these stones are frequently, either in error or by fraud, passed off as Oriental rubies ; but this is a deception that should be easy of detection, as they are deficient in hardness, and do not show dichroism ; the specific gravity of the ruby is also higher, and the crystalline system different.
Before the composition of the spinel was determined, and before the introduction of the delicate tests we now use to discriminate between gem-stones, there was no distinction made between the spinel and the Oriental ruby, a fact that accounts for the large number of gigantic stones being regarded during the middle ages as rubies, that are only spinels. De Lisle in 1783 is supposed to have been the first to distinguish between these different gem-stones. It is therefore to be supposed that previous to this date these spinels were honestly believed to be what they were represented, and that they were bought and sold in ignorance, and not with desire to defraud. The differences between the varieties of spinel and the corundum gems, are to a casual observer so slight that it is little to be wondered at that early traders in these gems should have been deceived.