Spinel labours under the serious disadvantage of being overshadowed at almost all points by its opulent and more famous cousins, sapphire and ruby, and is not so well known as it deserves to be. The only variety which is valued as a gem is the rose-tinted stone called balas-ruby (Plate XXVII, Fig. 3), which is very similar to the true ruby in appearance; they are probably often confused, especially since they are found in intimate association in nature. Spinels of other colours are not very attractive to the eye, and are not likely to be in much demand. Blue spinel (Plate XXVII, Fig. 4) is far from common, but the shade is inclined to steely-blue, and is much inferior to the superb tint of the true sapphire. Spinel is very hard and eminently suitable for a ring-stone, but is seldom large and transparent enough for larger articles of jewellery. Spinel is an aluminate of magnesium corresponding to the formula MgAl_{2}O_{4}, and therefore is closely akin to corundum, alumina, and chrysoberyl, aluminate of beryllium. The composition may, however, vary considerably owing to the isomorphous replacement of one element by another; in particular, ferrous oxide or manganese oxide often takes the place of some magnesia, and ferric oxide or chromic oxide is found instead of part of the alumina. When pure, spinel is devoid of colour, but such stones are exceedingly rare. No doubt chromic oxide is responsible for the rose-red hue of balas-ruby, and also, when tempered by ferric oxide, for the orange tint of rubicelle, and manganese is probably the cause of the peculiar violet colour of almandine-spinel. It is scarcely possible to define all the shades between blue and red that may be assumed by spinel. Stones which are rich in iron are known as pleonaste or ceylonite; they are quite opaque, but are sometimes used for ornamental wear. The name of the species comes from a diminutive form of σπῖνος, a spark, and refers to the fiery red colour of the most valued kind of spinel. It may be noted that the Latin equivalent of the word, _carbunculus_, has been applied to the crimson garnet when cut _en cabochon_. Balas is derived from _Balascia_, the old name for Badakshan, the district from which the finest stones were brought in mediæval times. [Illustration: FIGS. 73, 74.—Spinel Crystals.]