The fullest account of the adamas as a stone is found in Pliny, who says it exceeds in value all human things, and its use was confined to kings, and to few even of them. He mentions six varieties, the most remarkable being the Indian and Arabian, of such unspeakable hardness that when struck with a hammer even the iron and anvil were torn asunder—“ita respuentes ictum, ut ferrum utrinque dissultet, incudesque etiam ipsi dissiliant.” It also resisted the fire, and could only be subdued and broken down when dipped in fresh warm goat’s blood. Similar fables continued to prevail during the Middle Ages, and even yet have hardly vanished from popular belief. As an ornamental stone it was highly esteemed during the early times of the Roman empire, as some scandalous stories recorded by Juvenal testify, though only stones with naturally polished faces could be used. This fact is proved not only by the words of Seneca—“nec secari adamas aut caedi vel deteri potest”—and others, but f¢om specimens of diamonds set in gold, with no artificial polishing, which have come down both from classic times and from the Middle Ages. This unworkable character long greatly limited both its use and its value; and the more highly coloured rubies, and even emeralds and sapphires, were often preferred to it. It was only after Ludwig van Berquen (or Berghem, as he is often named) in 1476 discovered the mode of cutting and polishing it, that the diamond slowly regained the, first place among gems. Even in the 16th century (1550), Benvenuto Cellini (Trattato dell y orificerio, cap. i.) assigns it only the third rank in value, estimating a perfect ruby of one carat weight as worth 800 scudi d’oro (each equal to about 4s.), a similar emerald at 400, an equal diamond at 100, and a sapphire at 10 scudi. In the same century the use of the diamond for cutting glass and engraving gems seems also to have become known.

The diamond always occurs in crystals of the tesseral or cubical system. Its most frequent forms are the octahedron, or double four-sided pyramid (fig. 1), the rhombic dodecahedron with twelve faces (fig. 2), and others with twenty-four (fig. 3), and forty-eight faces

(fig. 4). The first form is most common in stones from India, the second in those from Brazil. Cubes also occur, but are rare, whilst the icositetrahedron has not been observed. Hitherto the diamond has been described as hemihedric, but Sadebeck from his own and G. Rose’s researches shows it to be holohedric (in the Berlin Monatsberichte, Oct. 1876). The faces are often curved, strongly striated, or marked by stair-like inequalities, hiding the true form. Many of the crystals also are round almost like spheres (fig. 5), or the smaller ones like grains of sand. This does not arise, however, from attrition during transport by water, but is the original shape of the stones. Macles, or twin-crystals, specially of two octahedrons, are common, and the striae due to this structure appear even on the polished facets. The diamond has a perfect cleavage parallel to the faces of the octahedron, and breaks readily both in this and other directions. Contrary to the old and still common opinion, it is rather brittle, and is easily injured by a slight blow or fall. Its hardness—10 in the mineralogical scale—far surpasses that of all other known stones, and was used even by the ancients to discriminate it from other gems. In specific gravity, 3’52 (or 3’515 to 3’525), it is considerably higher than rock crystal, but nearly the same as the topaz, which may thus be mistaken for it. According to Fizeau, it has its greatest density at - 42° ’ 3 C., and below this begins to expand, a property seen in very few other solid bodies. Its expansion by heat is very small, the volume from the freezing to the boiling point of water only rising from 1 0 to 1’00000354. By friction it becomes positive electric. The so-called compact diamond or carbonado of the stone polishers, found as round grains or masses of one or two pounds weight in the washings near Bahia, of a brownish black colour and sp. g. = 3 ’ 012 to 3 ’ 416, is porous diamond mixed with a small amount of other matter.

The optical properties of the diamond are also very remarkable. The purest stones, or those of the first water are highly transparent and colourless. But more generally it is less transparent, and shows various tints, specially white, grey, or brown; more rarely blue, red, yellow, green; and very seldom black. Such stones, when the colours are pure, are often highly valued. It is also distinguished by its brilliant adamantine lustre. Newton, two centuries ago, remaιked its high refractive power, and from this conjectured that it was a substance of a peculiar nature. The index of refraction is 2’4135 for the red rays, 2’4195 for the yellow, and 2’4278 for the green. This high refractive power, and the strong reflection at both surfaces, render it seldom completely transparent, but give it the high lustre for which it is valued as an ornament. They also produce the numerous internal reflections seen in the interior of cut stones, all the rays of light falling on the posterior surfaces at angles above 25° being totally reflected. Like all crystals of the same system it possesses only simple refraction, but Dr (Sir David) Brewster found that many showed traces of double refraction by their action on polarized light. This he ascribed to a peculiar tension produced in the interior of the stone during its formation, and a somewhat similar explanation is still adopted.

In a history of gems published early in the 17th century, Boetius de Boot conjectured that the diamond was an inflammable body. Robert Boyle, who in 1664 described its property of shining in the dark, or phosphorescing after being exposed to the light of the sun, a few years later observed that a part of it was dissipated in acrid vapours when subjected to a high temperature. This combustibility of the diamond was confirmed in 1694 and 1695 by experiments with a powerful burning glass or lens made in the presence of Cosmo III., grand duke of Tuscany, by the Florentine Academicians. The experiment of the combustibility of the diamond when freely exposed in a strong heat has been often repeated, and its true character was proved by Lavoisier, who determined that the product was carbonic acid gas. Sir George Mackenzie converted iron into steel by powdered diamonds; whilst Mr Smithson Tennant showed that the carbonic acid produced corresponded to the oxygen consumed. No doubt, therefore, now remains that the diamond is only pure carbon in the crystallized condition, and like it insoluble in acids.

In regard to the action of heat on the diamond, various experiments have been made. Before the blowpipe it is infusible, and closely packed in powdered charcoal it can resist a very high temperature. But when oxygen is present it burns slowly at a temperature usually given at about that of melting silver. Gustaf Rose lately found that when air is excluded diamonds exposed to a temperature at which pig-iron melts, or to the strongest heat produced in the porcelain kiln, undergo no change; but at a higher temperature, like that at which bar-iron melts, they begin, whilst retaining their form, to be converted into graphite. He further observed that when diamonds and graphite were exposed together in the same muffle, foliated graphite'was far more difficult to burn than the diamond, but compact graphite was consumed more readily. In the current of air the diamond gradually became smaller and smaller, but retaining its brilliancy till it finally vanished.

The faces also during burning became marked with peculiar triangular hollows, with their sides parallel to the edges of the octahedron. Seen in a strong light they appear as faces of an icositetrahedron, whilst other regular triangular impressions on the faces of natural crystals of diamond are produced by faces of the dodecahedron. (Rose, “Ueber das Ver halten des Diamants,” &c., in Berlin Monatsberichte, June 1872).

India is the oldest, and was long the most celebrated, or rather the only, source of diamonds. They have been obtained from a wide district on the eastern side of the Deccan, extending from the Pennar river in 14° N. lat. to near the Sone, in Bundelkund, in 25° N. lat. In the south the chief mines were at Cuddapah, Karnul, and Ellore, near the Kishna, in Madras presidency. In this district some of the largest Indian diamonds were obtained, Golconda, however, not being a mine, but a fortress where the diamonds were collected. There were other mines near Nagpore, and east at Sambhalpur, on the Mahanuddy, and north at Panna, in Bundelkund. At all of these the diamond was sought chiefly in recent deposits, beds of sand and clay, or in some places a ferruginous sandstone or conglomerate, but probably none of them the original matrix. Heyne states that the diamond has hitherto been found only in alluvial soil, or in the most recent rocks; and that the stones are not scattered through the whole of these beds, but confined to one rather harder than the rest. The upper stratum, of 18 inches, consists of sand, gravel, and loam; next there is a deposit of stiff black clay or mud, about 4 feet thick; and next the diamond bed, which is distinguished by a mixture of large rounded stones. It is from 2 to 2∣ feet thick, closely cemented together with clay. Sometimes this stratum is covered with calcareous tufa. Here shallow pits are excavated, of a few feet in diameter, in such spots as the practice of the workman may induce him to select; he sinks to a depth of a few feet, and searches the bed which he considers most promising for his purposes; and if he meets with little encouragement, he shifts his situation and proceeds elsewhere. Thus a great deal of the country may be turned to waste and neglected. The working was chiefly in the hands of certain tribes or castes, but was conducted on no regular plan, and afforded a very miserable livelihood. There has been little change since, and though mines still exist at Panna, Karnul, and a few other places, but comparatively few diamonds are found, and probably scarcely pay the expense of collecting them. Diamonds have also been long collected in Borneo, at Pontiana, near the southeast extremity of the island. They occur in a red clay along with gold and platina, and the rajah of Mattan is said to possess one weighing 367 carats, of the purest water, but uncut.

During the end of last and the beginning of the present century the supply of-diamonds chiefly came from Brazil. They were first recognized in 1727 in the province of Minas Geraes, where they had been long used by the negroes as counters in playing cards. The principal mines are still in that province near Diamantina (formerly Tejuco), and near Diamantino in Matto Grosso. Mines have also been recently worked in the province of Bahia Other localities are enumerated in the article Brazil. The diamonds are chiefly obtained from the Cascalho, a loose, gravelly deposit mixed with red clay, and containing large lumps of quartz and grains of gold. This rock is probably derived from the itacolumite, a quartzose variety of mica slate, or metamorphosed sandstone, on which it often rests, and in which diamonds are also said to occur. When first brought to Europe the Brazil diamonds were regarded as inferior to those from India, but without reason. Though the mines are strictly watched as Crown property, the produce is nut well ascertained. Martius estimated that in the forty-six years from 1772 to 1818 diamonds weighing about 3,000,000 carats, and worth £7,000,000, were exported. Mr Mawe stated the produce at 25,000 to 30,000 carats annually of rough diamonds, equal to 8000 or 9000 carats when reduced to brilliants. After his time it seems to have greatly decreased, the whole value from 1861 to 1867 being given at about £1,900,000; and the discovery of the Cape diamonds has further reduced the amount. The stones are mostly small, averaging little more than one carat, and very rarely exceeding twenty carats. The largest diamond from Brazil was long an uncut octahedron of 120 carats, but in 1854 a fine stone of 254¾ carats was sent to London. It was an irregular dodecahedron, but of brilliant lustre and with no flaws. Since cut it weighs about 124 carats, and is known as the “Star of the South.”

Diamonds occur in other parts of America, having been found in the Sierra Madre, south-west of Acapulco in Mexico; and a few also in Georgia and North Carolina. They have also been obtained in California, but all small (under 2 carats); and in the district of Arizona, where one is mentioned of 3 carats.

In 1829 diamonds were discovered on the European side of the Ural mountains in the gold washings near the iron mines of Bissersk. Engelhardt conjectured that they were derived from a dolomite rock, but others state that it is mica slate like that of Brazil. Only about seventy were found in the first twenty years, and all of them small, the largest weighing under 8 carats. The only other European locality is at Dlaschkowitz, in Bohemia, where a single diamond was found in the sand containing pyropes,—the one said by Murray to have been picked up in a brook in Ireland being very doubtful.

Not more important are those from Australia, where they were found as early as 1852, and again in 1859, on the Macquarie river. In 1869 they were discovered in the Mudgee, near one of the tributaries of the Macquarie, by gold-diggers, and worked for a time pretty extensively. They lie there in old river drift covered by basalt said to be of Pliocene age. They occur in a similar position in the Bíngera diamond field. In both places they are sparingly distributed and small, the largest mentioned being under 6 carats.

Far more important are the diamond fields of South Africa. In 1867 a Dutch farmer obtained from a boer a bright stone which his children were using as a plaything. This stone was sent to the Cape, where its true nature as a diamond was recognized, and subsequently forwarded to the Paris exhibition and sold for £500. This valuable discovery soon led to further researches, and diamonds were obtained from various places near the Orange and Vaal rivers in Griqua Land West. They were first collected by washing recent alluvial or supposed lacustrine deposits, apparently the detritus of rocks in the vicinity, that are spread over the lower river valleys, but are now rather sought for in “pans,” or “pipes,” of a circular form running down into the inferior strata, or shale, and filled with a peculiar igneous rock, named diabase, or gabbro, often much changed near the surface. Throughout this rock l which has been penetrated to a depth of from 100 to 200 feet, diamonds are disseminated weighing from over 150 carats down to the 100th of a carat, or less. Many are entire, well-formed crystals, but a large proportion are broken and isolated fragments. Hence it has been inferred that the rock in which they now occur is not the matrix or mother rock in which they were originally formed, but that the “pipes” are rather channels by which volcanic matter has made its way to the surface, bringing the diamonds along with it from some inferior deposit. However this may be, diamond-digging has become a regular branch of industry to a large population; and it is probable, though no very accurate estimate can be formed, that nearly fifteen million pounds sterling worth of diamonds have been obtained from this district since their discovery. The largest diamond from the Cape we have seen mentioned is the Stewart, of 288∣ carats, found on the Vaal river in 1872. It was an irregular octahedron of the purest water, and l¾ inch in diameter, and is of a light yellow since cut.

There has been much speculation regarding the mode of origin of these gems, but hitherto leading to no certain result. Newton conjectured that the diamond was “an unctuous substance coagulated;” Jameson thought it might be a secretion from some ancient tree, like amber; and Brewster also traced it to a vegetable source. Lavoisier, Guyton-Morveau, and others observed black specks when diamonds were burned, which were considered as uncrystallized carbon. Petzhold, in 1842, also supported this view, affirming that he had found vegetable cells in the ashes of diamonds. Goeppert, in his Haarlem Prize Essay, in 1863, supported the same view, both from supposed plant tissues and from other inclosures in diamonds, but admitted that the evidence was not free from doubts. Liebig and others have explained its origin by a slow process of decomposition in a fluid rich in carbon and hydrogen. On the other hand, the occurrence of the diamond in the itacolumite or mica slate, and more recently in or near igneous rocks, as at the Cape, has tended to favour the view that it owes its origin to heat or metamorphic action, as is the case with graphite. But this, as graph , 'te also shows, does not preclude the idea that originally it may have been, like amber, some peculiar vegetable product, subsequently altered and crystallized. It may here also be mentioned that all attempts to produce diamonds artificially have hitherto failed.

Diamonds are chiefly used and valued as ornamental stones, and for this purpose they are cut in various forms according to the original shape of the crystals. It is probable that the Indians knew some method of doing this at an early period, and it is said there were diamond-polishers in Nuremberg even in 1373. Berghem of Bruges has the credit of having first used, in 1456, their own powder for this purpose. He found that by rubbing two diamonds on each other their surfaces were polished and facets formed, and acting on this hint, he employed diamond powder and a polishing wheel. His countrymen continued to follow out the art with great success, but some two centuries ago the English cutters were the more celebrated. The trade then reverted to Holland, but is again returning to Britain, where many of the finest stones are cut·. The method has undergone little change, and is still chiefly effected by the hand, partly by rubbing one stone on another, partly by a wheel and diamond powder. Where there are flaws or large pieces of value to be removed, they are occasionally cut by iron wires armed with the powder, or split by a blow of a hammer and chisel in the direction of the natural cleavage. The latter is, however, a dangerous process, as the diamond is very brittle, and many valuable gems have been thus destroyed. When reduced to a proper form, the facets are polished on a lapidary’s wheel. The process demands not only great skill but much time and labour. The period required to reduce a stone of 24 or 30 carats to a regular form extended formerly to at least seven or eight months of constant work, and in the case of the Pitt diamond two years were needed; but the time is now greatly...