DIAMOND. The diamond has always been regarded as the most valuable of the gems, and, consequently, as the most valuable production of the Mineral world; a superiority which it derives from its very high lustre, its transparency, and hardness. The first quality arises from its greater refractive power, which is such as to cause all the light to be reflected which falls on it at an angle of incidence greater than 24i degrees ; and it is capa ble of being rendered still more brilliant by its surface being cut into facets, which multiply the reflections of light. From its hardness, too, its lustre remains unin jured : this hardness is such, that it can be cut, or rather worn down, only by rubbing one diamond against another, and is polished only by the finer diamond powder.
This substance is found• in India, in the districts of Visapore and Golconda, and likewise in Bengal, and in Brazil in South America. It is not found in its original situation, but in the beds of streams, or in a loose ferruginous sand beneath the soil. The Brazilian diamonds are infe rior in transparency and purity to the Oriental.
The diamond is found crystallized, being either in perfect crystals, or in fragments often encrusted with a hard coating. The usual form is an octahe dron, composed of two four-sided pyra mids joined by the base, the faces being somewhat convex. Of this form there are some modifications ; the angles being replaced by triangular faces, so as to give rise to a dodecahedron of twenty-four faces, likewise a little convex. These are the crystallizations of the Oriental dia mond. The Brazilian is generally a do decahedron, with rhomboidal faces. These crystalline forms are often imperfect,pro bably frotn the attrition which they have suffered, and frequently the fragments are altogether indistinct.
The diamond is colourless, or tinged of various shades of white or grey, and sometimes also, though more rarely, of brown, green, yellow, blue, and red, fre quently with darker coloured spots. It is generally transparent, though not perfect ly so, and has the property of single re fraction ; its fracture is lamellated, and it can be split by striking it in the direction of the plates. Its specific gravity is from 3500 to 3600.
The diamond is phosphorescent, or, when it has been exposed to the light, is luminous in the dark. It is rendered electrical by rubbing, the electricity be ing positive.
From the qualities of the diamond it was long ranked with the other gems, and considered as analogous to them in its chemical construction. Newton, by a happy application of a physical princi ple, conjectured that it was an inflam mable substance. Transparent bodies, which are uninflammable, refract light nearly in the ratio of their densities, while those which are inflammable have refractive powers which are greater than their densities: and the diamond having this great refractive power led Newton to conclude, that it " probably is an unc tuous substance coagulated." (Optics, Book II. Prop. 10.) In 1695 experiments had been made at Florence, which prov ed the diamond to be dissipated by the intense heat in the focus of the power ful burning lens of Tschirnausen. After ward, in experiments made at Vienna, it was found, that in the heat of a furnace, diamonds lost weight, and, if exposed for a sufficient length of time, entirely dissappeared, while the ruby and other gems, exposed to the same heat, remain ed unaltered. At a later period Benet exposed diamonds to heat, enclosed in balls of porcelain clay, in various ways, and always found that they were dis sipated by exposure to a strong heat. These facts at the same time appeared in contradiction to the common practice of the jewellers who expose diamonds which are foul to a strong heat, imbedded in charcoal, to make them clear. An ob servation of Macquer first threw light on this subject. He took notice, that While the diamond was exposed to a strong heat under a muffle, and while it was losing weight, it was luminous, and appeared to burn, a fact which he verified by subsequent experiments. It cannot be doubted, therefore, that in the experiments of Darcet air had been admitted to the diamonds, from rents in the porcelain clay balls in which they were inclosed, and that in the method of jewellers they are more effectually protected from the action bf the air, by the charcoal dust with which they are surrounded.