Noble Corundum of other colors than those of blue and red is not of abundant occurrence nor is it ordinarily as highly prized as are the sapphire and ruby. Colorless sapphire or leucosapphire is sometimes used as a substitute for the diamond, from which it can readily be distinguished by its lower hardness and higher specific gravity.
Certain specimens of both sapphire and ruby, but especially the former, exhibit when lighted a six-rayed star. This appears as beams of light, radiating from a center, which changes in position as the stone is turned. Such stones are called star or asteriated sapphires or rubies, and are highly prized. They are usually cut with rounded surface, as this best brings out the figure. The cause of the star-shaped figure is generally supposed to be the presence of countless microscopic cavities in the stone, which are arranged parallel to the faces of a six-sided prism. The total reflection of the light from these causes the star. Others think that multitudes of twining lamellæ cause the appearance.
Sapphire is a word which is the same in nearly all languages. In Chaldean, Hebrew, Greek and Latin it has the same form as in modern tongues. This fact testifies to the ancient use of the stone. In early times sapphire was believed to be a destroyer of poison, so that if put into a glass with a spider or venomous reptile it would kill it. It was regarded also as a remedy against fevers.
SPINEL.
The group of Spinel includes in mineralogy a number of species of different though analogous composition. The Spinel employed as a gem is almost wholly confined to the magnesium aluminate, having the percentage composition alumina 71.8 and magnesia 28.2. This is usually of a red color, different shades giving gems known by different names as follows: Deep red, spinel-ruby; rose-red, Balas ruby; yellow- or orange-red, rubicelle; violet red, almandine ruby. Spinel is thus known among gems chiefly as a relative of the ruby, and this sort of Spinel will first be considered.
The Spinel rubies differ from the true or corundum rubies in hardness, specific gravity and system of crystallization. The hardness of Spinel is 8, or about that of topaz, and the specific gravity 3.6. It is thus neither as hard nor as heavy as corundum ruby. Again, the system of crystallization differs. Spinel crystallizes in the isometric system and is usually found in the form, of octahedrons, while corundum ruby is hexagonal in crystallization. (See colored plate in November number.) Spinel is singly refracting in polarized light and corundum doubly refracting. Spinel ruby is infusible before the blowpipe, but on heating undergoes a curious series of changes in color which are quite characteristic. The red changes first to brown, and then becomes black and opaque, but on cooling the black changes to green, then becomes nearly colorless and finally the stone resumes its original red color. As a small percentage of chromium is usually found by analysis to exist in ruby Spinel, its color is generally considered to be due to this ingredient. While the Spinel ruby is considered of less value than the corundum ruby and is sometimes by fraud or error substituted for the latter, it yet has a definite value as a gem and is sold under the name of Spinel ruby or some of its varieties. This value is usually reckoned at about half that of the corundum ruby, although variations in quality of the stones, as well as changes in demand, cause differences of price. Thus Emanuel mentions a Spinel ruby of good quality weighing 40 carats, which in 1856 was sold for two thousand dollars, but which in 1862 brought at public auction only four hundred dollars. In 1866, however, it was again sold for twelve hundred dollars. The Spinel ruby of the French crown jewels, weighing 56 carats, was in 1791 valued at ten thousand dollars.
Not only is Spinel ruby related to corundum ruby in color and use, but the two are frequently associated together in nature. The gem gravels of Ceylon, Siam, Australia and Brazil contain both kinds of rubies, and the ruby mines of Upper Burmah, where the corundum ruby occurs in a crystalline limestone, produce also large quantities of Spinel rubies. Spinel rubies also come in large quantity from Badakschan, in Afghanistan, near the river Oxus, the name of Balas rubies, by which they are often known, being said to be derived from Beloochistan, or Balakschan. The Persians have a tradition regarding these mines that they were disclosed by an earthquake which rent the mountain in twain. The localities above mentioned furnish nearly all the Spinel rubies of commerce. A few have been found in North America at Hamburgh, New Jersey, and San Luis Obispo, California. But these localities have never afforded any appreciable supply. No Spinel rubies of great size are known. Bauer mentions as the largest known, two cut stones, one of 81 carats and the other 72½ carats, exhibited at the London Exposition of 1862. The King of Oude is said at one time to have possessed a Spinel ruby the size of a pigeon’s egg.
Spinel occurs in many other colors besides red, such as orange, green, blue and indigo, as well as white and black. Occasionally colorless Spinels occur, and as they cannot be distinguished by their behavior in polarized light from the diamond, it is sometimes sought to substitute them for the latter. They can be detected at once, however, by their inferior hardness. While Spinels of any color, if transparent and free from flaws, make desirable gems, the only colors found in sufficient quantity outside of the red to make an appreciable supply are the blue and the black. The blue Spinels resemble the sapphire in color, though they are somewhat paler. They come chiefly from Ceylon and Burmah, where they occur together with the ruby Spinel. The black Spinel is known as Ceylonite, or pleonaste, and is also obtained chiefly from Ceylon, although occurring of quality suitable for cutting at Mount Vesuvius in Italy.
Like the ruby, Spinel can be made artificially, the process being to heat a mixture of alumina and magnesia with boracic acid, and if the red color is desired, a little chromium oxide.
The Spinel ruby seems to have been known to the ancients equally with the corundum ruby, and the two were probably often confounded. The natives of India call the Spinel the pomegranate ruby and believe to this day that it possesses valuable medicinal properties.