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 MgAl2O4, 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 ottivos, 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 mediaeval times.
Spinet, like diamond, belongs to the cubic system of crystalline symmetry, and occurs in beautiful octahedra, or in flat triangular-shaped plates (Figs. 73, 74) the girdles of which are cleft at each corner, these plates being really twinned octahedra. The refraction is, of course, single, and there is therefore no double refraction or dichroism ; this test furnishes the simplest way of discriminating between the balas and the true ruby. Owing to isomorphous replacement the value of the refractive index may lie anywhere between 1.716 and 1736. The lower values, about 1.720, correspond to the most trans-parent red and blue stones; the deep violet stones have values above 1730. Spinel possesses little colour-dispersion, or ‘fire.’ In the same way the values of the specific gravity, even of the trans-parent stones, vary between 3.5 and 3.7, but the opaque ceylonite has values as high as 4.1. Spinel is slightly softer than sapphire and ruby, and has the symbol 8 on Mohs’s scale, and it is scarcely inferior in lustre to these stones. Spinel is easily separated from garnet of similar colour by its lower refractivity. Spinels run from 10S. to £5 a carat, depending on their colour and quality, and exceptional stones command a higher rate.
Spinel always occurs in close association with corundum. The balas and the true ruby are mixed together in the limestones of Burma and Siam. Curiously enough, the spinel despite its lower hardness is found in the river gravels in perfect crystals, whereas the rubies are generally waterworn. Fine violet and blue spinels 0ccur in the prolific gem-gravels of Ceylon. A large waterworn octahedron and a rough mass, both of a fine red colour, are exhibited in the Mineral Gallery of the British Museum (Natural History), and a beautiful faceted blue stone is shown close by.
The enormous red stone, oval in shape, which is set in front of the English crown, is not a ruby; as it was formerly believed to be, but a spinel. It was given to the gallant Black Prince by Pedro the Cruel after the battle of Najera in 1367, and was subsequently worn by Henry v upon his helmet at the battle of Agincourt. As usual with Indian-fashioned stones it is pierced through the middle, but the hole is now hidden by a small stone of similar colour.
The British Regalia also contains the famous stone called the Timur Ruby or Khiraj-i-Alam (Tribute of the World), which weighs just over 352 carats, and is the largest spinel-ruby known. It is uncut, but polished. Its history goes back to 1398, when it was captured by the Amir Timur at Delhi. On the wane of the Tartar empire the stone became the property of the Shahs of Persia, until it was given by Abbas I to his friend and ally, the Mogul Emperor, Jehangir. It remained at Delhi until, on the sack of that city by Nadir Shah in 1739, it, together with immense booty, including the Koh-i-nor, fell into the hands of the conqueror. Like the great diamond, it eventually came into the possession of Runjit Singh at Lahore, and on the annexation of the Punjab in 1850 passed to the East India Company. It was shown at the Great Exhibition of 1851, and after-wards presented to Queen Victoria.
Mention has been made above (p. 121) of the blue spinel which is manufactured in imitation of the true sapphire. The artificial stone is quite different in tint from the blue spinel found in nature.