DIAMOND. This crystalline gem, on as munt of its high lustre and extreme hardness 1 a 3 always been regarded as the most valuable if the precious stones. The diamond usually occurs in imbedded octahedral crystals in al luvial ground, in the East Indies, Brazil, and he Ural mountains. It is commonly colour less or grayish, but sometimes green, yellow, Fed, brown, blue, and black : the two last mentioned colours are the rarest. Its lustre is adamantine ; refraction single; transparent, but sometimes rendered opaque by foreign matter. It is harderthan any other substance, and can be cut or worn down only by rubbing one diamond against another; and it is po lished by the friction of portions of the gem itself reduced to powder. It is broken with out difficulty. The specific gravity of the dia mond is 3.52. When rubbed, it phosphorises and becomes positively electrical. When heated, without the contact of air, it suffers no change ; but, if ignited in contact with it, it is totally converted into carbonic acid gas.
For ornamental purposes diamonds are cut into two shapes, namely, rose diamonds and brilliants. The weight and consequently the value of diamonds is estimated in carats, each of which is equal to 3.166 grains.
It was conjectured by Newton that the dia mond is a combustible body; but the proof of its being absolute carbon in a crystalline form was developed by slow degrees by the Floren tine Academicians in the seventeenth century, by Lavoisier a century later, and by many chemists in the present century.
The pencil diamond used in cutting glass is a small fractured piece of diamond. The part of the diamond used is of a trapezoidal shape, weighing about the sixtieth part of a carat, and is set in a wooden handle.
The diamond-mines of Cuddapah in the East Indies, near Madras, instructively show the mode in which these gems are procured. These mines have, it is said, been worked for several hundred years with various success. The places in which diamonds have hitherto been found consist either of alluvial soil or of rocks of the latest formation. The mines are pits of small depth. The diamonds are not scattered through the whole of the mine from the surface to the greatest depth, but occur in beds always harder than the adjacent soil, and usually not exceeding one or two feet in thickness. Dr. Heyne, who carefully exam ined these mines, has given in his statistical tracts the following description of one: The uppermost, or superficial stratum, consists of ' sand or gravel, mixed with a small proportion of loam. Its thickness scarcely exceeds a foot and a half. Immediately under it is a bed of stiff blueish or black mud, similar to what are seen in places that have been inundated ; it is about five feet thick, and contains no stones. The diamond bed comes next, and is easily distinguished from the incumbent bed by the great number of large rounded stones which it contains. It is about two feet or two and a
half feet thick, and is composed of large round stones, pebbles, and gravel connected together by clay.' The contents of this bed are put into a cistern about eight feet square and three feet deep, when water is poured into it, which separates the clay and loamy parti cles, leaving the gravel and small stones at the bottom. These being removed are thinly spread upon a smooth surface, about twenty feet square, of hardened clay, and six or seven people examine the whole carefully several times. At first they pick out the large stones ; at the second and subsequent examinations the smaller gravel is carefully turned over by hand, while they watch for the spark from the diamond, which invariably strikes the eye.' In 1845 the public journals gave frequent extracts from Brazilian correspondence relat ing to the discovery of a new diamond mine in that country. The locality was the Assu era, a desert part of the province of Bahia. At this spot gold was found a few years earlier ; and among the many hundreds who flocked to work the gold veins, some dis covered the presence of diamonds. No sooner was the discovery made, than the gold was abandoned, and the more precious diamonds sought for ; and the accounts seem to convey the information that the store of these gems was very large. One ship is said to have car- , ried to Europe diamonds to the value of I 00:000/.
The commercial value of diamonds is a very curious feature. In the first place they are estimated by weight. A weight called a carat (equal to rather more than three grains troy) is taken as an unit, and is divided into halves, quarters, eighths, and sixteenths, to give fractional parts. Then the quality and form of the diamond are examined, and a price per carat fixed from all these circumstances. The larger the diamond the greater is the price per carat given for it. Under five carats, the price varies from thirty shillings to thirty guineas per carat: but very large diamonds, like chefs d'auvre among pictures, obtain a price limited only by the competition of the small number of persons able to purchase them. The Russian diamond, the Pitt dia mond, and the Pigott diamond, are gems which have obtained notoriety on account of their large size and enormous value. The celebrated Koh-i-noor, or Mountain of Light, is an Indian diamond which has lately fallen into the hands of the English at Lahore, and has been brought to this country as the pro perty of Queen Victoria. It weighed 900 carats when rough, but this is reduced to 279 by cutting and polishing. As to its value, guesses have varied from half a million to three millions and a half sterling: shewing how vague are all attempts to estimate such rarities. It is said that Her Majesty intends to deposit this gem at the Industrial Exhibi tion.