Diamond (a contraction of adamant, from Gr. a privative, sauav, to subdue), a gem so named on account of its extreme hardness, for which, and its brilliancy and beautiful play of prismatic colors, it excels all others. The diamond is pure crystallized carbon, and has a specific gravity slightly varying, according to the different qualities, as follows: Brazilian, colorless, 3.444; Brazilian, yellow, 3.519; oriental, colorless, 3.521; oriental, green, 3.524; oriental, blue, 3.525. Its hardness, according to an artificial standard scale, is 10, greater than that of any other known substance, that of corundum being 9, and that of quartz 7. The primitive form of the crystal, and that into which the secondary forms may be converted by cleavage, is the regular octahedron. The faces of the crystals are often convex, and the edges curved. The cleavage planes greatly facilitate its cutting, and also present the most brilliant natural surfaces. The gem is not acted upon by acids or alkalies, and when air is excluded may be heated to whiteness without injury.

When exposed to the heat of a powerful galvanic battery, it fuses and is converted into a mass resembling coke, its specific gravity being sometimes reduced to 2.68. Its combustion may be effected in the open air or in oxygen gas, but it requires a very intense heat, which is scarcely estimable from the difficulty of using apparatus. Its combustibility was first proved by the Florentine academicians in 1G94, by subjecting it to the solar rays concentrated in the focus of the large parabolic reflector made for Cosmo de' Medici, when it burned with a blue, lambent flame. - The diamond is found in alluvial deposits which are worked for gold, sometimes attached to loose pieces of brown hematite, and sometimes in a conglomerate of quartz and chalcedony, cemented by ferruginous clay; but it is not certain in what geological formation it originated. Generally in regions where the diamond is found there also occurs a laminated granular quartz rock, called itacolu-mite, of talcose formation, and which in thin plates is more or less flexible.

According to M. Denis, it is found in Minas Geraes, Brazil, in two different deposits : one, called gargulho, which is composed of pieces of broken quartz, covered by a thin bed of earth; and the other, called cascalho, composed of quartz pebbles, united by ferruginous clay, and usually resting on talcose clays, the whole being debris from talcose rocks. The first is said to yield the finest diamonds, and both contain gold, platinum, and magnetic iron. The diamond mines of India have the same character as those of Brazil, the alluvial earth being a conglomerate of more or less tenacity, which requires to be broken up. The diamond was long known in Asia, in Hin-dostan, Borneo, Sumatra, and in the Ural mountains, before it was discovered elsewhere; the district from Cape Comorin to the bay of Bengal, including the famous mines of Gol-conda, furnishing the world until 1728, when the Brazilian mines were discovered. (See Diamond District.) The South African diamond fields were discovered through some children finding a diamond of 21 1/4 carats on the banks of the Orange river, in 1868. In 1869 the "Star of South Africa," of 83 1/2 carats, was found by a Griqua shepherd; and on the borders of the Vaal river several small stones were found early in 1870. Since then the search has increased, until now there are said to be more than 4,000 persons, over an extent of nearly 1,500 sq. m., chiefly in the valleys of the Orange and Vaal rivers and at their junction, between lat. 26° and 28° S., and Ion.. 24° and 26° E. The diamonds are found, from the surface to a depth of 70 ft., in an alluvial calcareous earth, with rolled pebbles of quartz, chalcedony, jasper, and garnets, and decomposed feldspathic and micaceous rocks, resting on carbonaceous shale.

The peculiarity of the African diamond is the great number of stones of 80 carats and upward, with a preponderance of 90 per cent. of yellow-tinted. The commercial effect of such a quantity of yellow diamonds coming at once on the market has been to depreciate their value in an extraordinary degree, large yellow stones being now but one fourth the price they were five years since; small stones are not quite one half their previous value. The whole of the South African diamond region, about 17,000 sq. m., was annexed in 1871 as a colony of Great Britain, under the title of Griqualand. Recently the Brazilian fields have ceased to be profitable, and many of the mines are abandoned, few retaining the full number of workmen; and even the South African fields are said to be declining. In the United States, diamonds have been discovered in Rutherford co., N. C., and in Hall co., Ga.; also at Paris mine, Franklin co., N. C.; and at the village of Manchester, opposite Richmond, Va.; in California, at Cherokee ravine, in Butte co.; at Forest hill, El Dorado co., one of 1 1/2 carat, and at French Corral one of 1 1/3 carat, have been found.

In Australia, they have been met with in the valley of the Turon, in the bed of the Macquarie river, at Victoria, and at Fremantle in Western Australia. - Diamonds are found of various colors, as well as colorless and perfectly transparent. The latter are most esteemed, and are distinguished as diamonds of the first water from their semblance to a drop of clear spring water. When of a rose tint and of clear water, they are also highly valued. A yellow shade is objectionable, as is a cinnamon color, a stone having these rarely being clear and sound. Next to the rose, a green color is the least objectionable; many very fine diamonds have this tint; and some are found of a bluish color, and some black. For the valuation of diamonds an arbitrary rule has been given, which is, however, little regarded in actual sales of the most costly of these gems. Purchasers for such being few, the only real rule adopted, as in the sale of many other commodities, is to demand the highest price there is the least probability that one may be induced to pay. The mere statement of the rule is sufficient to show its indefiniteness. It is to multiply the square of the weight in carats by a sum varying according to the state and quality of the stone.

If clear and of good shape, this sum was £2; if perfect and well cut, £6 or £8 for the brilliant or rose, but a lower figure for the table. The rate is now $50 in place of the £2 above, and a specimen brilliant is worth $200. For diamonds of moderate size the rates vary as little as those of exchange between countries. They follow from the natural proportions in which diamonds are found. Diamonds weighing over 10 carats have a higher proportional theoretical value than the smaller sizes; yet the latter can commonly be sold at higher proportional rates, on account of the few purchasers for those of large size. In the great sale of jewels in London in 1837, on the distribution of the Deccan booty obtained by the army of the marquis of Hastings, the splendid Nassuck diamond, weighing 357 1/2 grains, and of the purest water, brought only £7,200. In December, 1858, £33,000 was paid for a stone weighing 61 carats, and £15,000 for a pair of drop-shaped diamonds for earrings. The finest gems of commerce are now in great part supplied by the old jewels of Portuguese, Spanish, French, and English families, the proportions from each nation being in the order named; and the best market for them is now the United States. - The origin of the art of cutting diamonds in a scientific manner is ascribed to Louis Berquen in 1456, who established a guild of diamond cutters at Bruges about the year 1470. Three large rough stones were intrusted to him by Charles the Bold, duke of Burgundy, for the cutting of which he received 3,000 ducats.

Diamond cutting was for a long time a monopoly in Holland, and the business is at the present day mostly confined to Amsterdam. The process, which consists of grinding down the surfaces as well as cutting, is slow and tedious, and being done by hand, occupies for a single stone the continual labor of months. The Pitt diamond required two years for the completion of the process. Two diamonds are employed, each cemented into the end of a stick or handle. The stones are then rubbed together with a strong pressure, being held over a metal box having a double bottom, the upper one perforated with small holes, through which diamond dust falls. This is afterward carefully collected, mixed with vegetable oil, and used for polishing the gem upon a revolving cast-iron disk. When a large piece is to be removed from the stone, it is sometimes cut off by means of a steel wire covered with diamond powder, and sometimes by the use of a chisel and hammer, though in this way there is danger of destroying the stone. The workmen should understand perfectly the position of the cleavage planes, as it is only upon them that pieces can be removed by the chisel. The forms usually adopted in cutting the diamond are the brilliant, the rose, and the table. The first shows the gem to the best advantage.

It is composed of a principal face called the table, surrounded by a number of facets which are cut upon that part of the stone which shows above the setting, and which is called the bezil. The greatest circumference forms the girdle, and below this is the pavilion, which should have a depth equal to one half the diameter of the gem at the girdle. The pavilion is terminated by a small facet, called the culet, which may be either square or octagonal. As the brilliant is the most economical of material, and shows the stone most advantageously, it is usually preferred to any other. The rose, which is very brilliant, is flat below and cut into facets entirely over the upper surface. The table is least beautiful, and is used mostly in India for thin stones with a large surface, which are ornamented by being cut into facets at the edges. - Among the most celebrated diamonds known, that obtained by Mr. Pitt, governor of Madras, is perhaps one of the finest and most perfect. It is known as the regent. Its weight before cutting was 410 carats, and by this process, which occupied two years, it was reduced to 136 carats, and was purchased by the regent duke of Orleans in 1718 for $675,000. Its present value is estimated at $1,000,000. It was placed by Napoleon in the hilt of the sword of state, and was captured by the Prussians at Waterloo. A splendid diamond found in Brazil some years ago, and carried to France, is called the "Star of the South." It weighs in its rough state 254 1/2 carats.

Its general form is a rhomboidal dodecahedron, and upon its faces are impressions which appear to have been made by other diamonds, so that the whole was probably a group of diamond crystals. The famous diamond in possession of the king of Portugal is also from Brazil. If genuine, of which there is some doubt, its value, according to the rule of computation, should be $28,000,000, weighing as it does in the rough 1,680 grains. The famous Koh-i-noor or "Mountain of Light" is now in possession of the queen of England. This stone, interesting alike for its historical associations and for its intrinsic beauty, was, according to Indian tradition, obtained before the Christian era from one of the mines of Golconda. From the rajah of Oojein, who seems to have possessed it at the beginning of the Christian era, it passed to successive sovereigns of central India, and in the early part of the 14th century was added to the treasures of Delhi by the Patan monarch Aladdin. It remained in possession of the ruling families of the empire until the irruption of the Persian conqueror Nadir Shah, who saw it glittering in the turban of the vanquished Mohammed Shah, and proposing an exchange of head dress as a mark of friendship, bore it away with him, and gave it the name by which it is still known.

After the assassination of Nadir it passed through the hands of Ahmed Shah of Cabool to Shah Shujah, who paid it as the price of his liberty to his conqueror Runjeet Singh, the "lion of the Punjaub," in 1813. On the annexation of the Punjaub to the East India company's territory in 1849, it was stipulated that the Koh-i-noor should be surrendered to the queen of England, to whom it was accordingly delivered by the company, July 3, 1850. At this period its weight was 186 carats. It was exhibited at the crystal palace in London in 1851, where it attracted universal attention; but its display of colors was inferior to that of its glass model, and it was necessary to surround it with a number of vivid lights to develop its colored refractions. An examination was made by scientific gentlemen, among them Sir David Brewster, with reference to the propriety of recutting the gem. After obtaining the opinions of skilful cutters at Amsterdam, it was decided that the attempt should be made, and it was given in charge of Mr. Coster of that city, who afterward cut the Star of the South. The diamond so long in possession of the sultan of Matan, of the island of Borneo, is remarkable for its size and purity.

It weighs 367 carats, and should be worth at least $3,500,000. It is shaped like an egg with an indented hollow in the smaller end. It was discovered at Landak. The Orloff diamond, purchased for the empress Catharine II. of Russia, is about the size of a pigeon's egg, and weighs 195 carats. It is said to have formed the eye of a famous idol in a temple of Brahma at Pondicherry. A French deserter robbed the pagoda of this valuable stone. After passing through the hands of various purchasers, it came into the possession of a Greek merchant, who received for it from the empress $450,000, an annuity of $20,000, and a title of nobility. The Austrian diamond is of a beautiful lemon color, and cut in rose; its weight is 139 carats. Its value is less than it would be but for its color and the form in which it is cut, ranking as worth $500,000 instead of $750,000. The most valuable diamond found in the United States was picked up by a workman at Manchester, on the banks of the James river, opposite Richmond, in 1856. The locality is in the tertiary formation, and the diamond originally belonged, no doubt, to the gold region up the river. It is of curvilinear octahedral form, specific gravity 3.503, and weighs 23.7 carats.

It is lightly chatoyant, and would probably cut white; but an original flaw was increased by the rough treatment it received from those into whose hands it fell, so that its value was greatly deteriorated. - The process of collecting diamonds is similar to that of collecting gold in the alluvial deposits. The coarse gravel and rolled pebbles derived from the primary and metamorphic rocks form the lowest stratum among the sands and clays of the alluvium. This stratum, resting upon the surface of the rock, is the repository alike of gold and of diamonds. It is laid bare in the beds of the streams, when these cease to flow in the dry season, or are drawn off by sluices made for the purpose. From these beds, as well as from excavations in the bottom, the gravelly conglomerate or cascalho is removed, to be washed when convenient. This in Brazil is usually in the rainy season, and the work is done in a long shed, through which a stream of water is conveyed, and admitted into boxes in which the cascalho is washed. A negro works at each box, and inspectors are placed to watch the work, and to prevent the laborers from secreting the diamonds. It is the custom to liberate the negro who finds a diamond weighing 17 1/2 carats.

Dr. Beke, in a paper read at a meeting of the British association, relates that a slave in Brazil seeking for diamonds in the bed of a river broke with his iron bar through a crust of silicious materials, cemented together by oxide of iron, in which he discovered a bed of diamonds, which were afterward sold for $1,500,000. This immense quantity, being carried to England, so overstocked the market that few of the English houses were able to stand up against it. - Besides their use as ornaments, diamonds are applied to several practical purposes. An impure diamond, but very hard, and colored black, known under the name of bort, is used for arming the bits of the diamond drill (see Boring), and is also crushed to fine powder in a steel mortar, and used for coating the metallic disks employed by lapidaries for producing flat surfaces on precious stones of great hardness. The fine splinters are made into drills, for piercing small holes through rubies and other hard stones. The property possessed by the diamond of cutting glass is due not merely to its extreme hardness, but to the peculiarity of its crystallization in rounded faces and curvilinear edges.

The natural crystal only is suitable for this purpose. - The diamond exhibiting the physical properties of matter in their highest state of perfection, and proving after all to be of the simplest chemical composition, it has been a matter of no little scientific interest to study the peculiarities of its construction, and to determine if possible the secret processes by which nature has elaborated the most perfect gem from so homely a substance as charcoal. Its high value has stimulated these researches, in the hope of individual profit by its artificial production. But though more complicated forms of matter have been successfully reproduced, carbon has not yet been made to attain the simple perfection of the diamond, unless it be in crystals invisible to the naked eye; nor have we yet learned from what department of nature's works the material has been taken, that has been so beautifully perfected. The vegetable kingdom may have furnished it, after itself receiving it from the atmosphere, or it may have been unlocked from those repositories of carbon shut up from remote geological periods in the carbonic acid of the calcareous rocks, or from such collections of fossilized plants as are now seen in various stages of change to mineral substance.

But if the direct object of these researches has not been attained, the forces which have acted upon it to give to it some of its peculiarities have been partially determined, as also a previous condition in which it must have existed. Sir David Brewster, from the exhibition of polarized light around the minute and irregular cavities in diamonds, has concluded that the substance has once been in a soft state, and compressed in these parts by the expansive action of a gas or fluid contained in the cavities; and as various circumstances indicate that this softness was not the effect of either solvents or heat, he is of opinion that, like amber, the diamond is a vegetable substance, slowly consolidated into a crystalline form. The nearest approach to its reproduction has been in the experiments of M. Despretz. By long continued voltaic action, carbon free from every trace of mineral substance, prepared from crystallized sugar candy, was made to deposit microscopic crystals in black octahedrons, in colorless translucent plates, the whole of which had the hardness of the powder of the diamond, and which disappeared in combustion without leaving any perceptible residue.

Being, however, only in powder, it was impossible to isolate and weigh these crystals, or to determine their index of refraction and angles of polarization. It is said that a similar result has been obtained by decomposing a mixture of chloride of carbon and alcohol by galvanic currents continued for six months. - The principal English works on the subject are D. Jeffrey's "Treatise on Diamonds and Pearls" (8vo, London, 1750); J. Mawe's "Treatise on Diamonds and Precious Stones" (8vo, London, 1826); and "Diamonds and Precious Stones" by Harry Emanuel (12mo, London, 1867; New-York, 1873).