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Diamond

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DIAMOND, mineral universally recognized as chief among precious stones; it is the hardest, the most imperishable, and also the most brilliant of minerals. These qualities alone have made it supreme as a jewel since early times, and yet the real brilliancy of the stone is not displayed until it has been faceted by the art of the lapidary (q.v.) and this was scarcely developed before the year i 746. The consummate hardness of the diamond, in spite of its high price, has made it most useful for purposes of grinding, polishing and drilling. Numerous attempts have been made to manufacture the diamond by artificial means, and these attempts have a high scientific interest on account of the mystery which surrounds the natural origin of this remarkable mineral. Its phys ical and chemical properties have been the subject of much study, and have a special interest in view of the extraordinary difference between the physical characters of the diamond and those of graphite (blacklead) or charcoal, with which it is chemically iden tical, and into which it can be converted by the action of heat or electricity.

The name 'Ahaµas "the invincible," was probably applied by the Greeks to hard metals, and thence to corundum (emery) and other hard stones. According to Charles William King, the first undoubted application of the name to the diamond is found in Manilius (A.D. i6)—Sic Adamas, punctum lapidis; pretiosior auro, —and Pliny (A.D. ioo) speaks of the rarity of the stone, "the most valuable of gems, known only to kings." Pliny described six varieties, among which the Indian, having six pointed angles, and also resembling two pyramids (turbines, whip-tops) placed base to base, may probably be identified as the ordinary octahedral crystal (fig. I). The diamond (Yahalom) in the breastplate of the high priest (Ex. xxxix. I I) was certainly some other stone, for it bore the name of a tribe, and methods of engraving the true diamond cannot have been known so early. The stone can hardly have become familiar to the Romans until introduced from India, where it was probably mined at a very early period. But one or other of the remaining varieties mentioned by Pliny (the Mace donian, the Arabian, the Cyprian, etc.) may be the true diamond, which was in great request for the tool of the gem-engraver. Later Roman authors mentioned various rivers in India as yielding the Adamas among their sands. The name Adamas became corrupted into the forms adamant, diamaunt, diamant, diamond; but the same word, owing to a mediaeval misinterpretation which derived it from adamare (compare the French word aimant), was also applied to the lodestone.

Scientific Characters.

Diamond is almost always found in single crystals, which show no signs of previous attachment to any matrix; the stones were, until the discovery of the South African mines, almost entirely derived from sands or gravels, but owing to the hardness of the mineral it is rarely, if ever, water worn, and the crystals are often very perfect. The crystals belong to the cubic system, generally as suming the form of the octahedron (fig. I), but they may, in accordance with the principles of crys tallography, also occur in other forms symmetrically derived from the octahedron; e.g., the cube, the i 2-faced figure known as the rhombic dodecahedron (fig. 2), or the 48-faced figure known as the hexakis octahedron (fig. 3), or in combinations of these. The octahedron faces are usually smooth ; most of the other faces are rounded (fig. 4). The cube faces are rough with protruding points. The cube is sometimes found in Brazil, but is very rare among the South African stones ; and the dodecahedron is per haps more common in Brazil than elsewhere. The crystals are sometimes tetrahedral in aspect. There are also "twins" of diamond in which two octahedra (fig. 5) are united by contact along a surface parallel to an octahedron face ; sometimes they in terpenetrate. They are generally flattened along the plane of union. The crystals often display triangular markings, either ele vations or pits, upon the octahedron faces (fig. 6). They have probably been produced by the action of some solvent. The actual arrangement of the carbon atoms in the crystal has now been ascertained by means of X-ray investigations.

Diamond possesses a brilliant "adamantine" lustre, but this tends to be greasy on the surface of the natural stones and gives the rounded crystals somewhat the appearance of drops of gum. Absolutely colourless stones are not so common as cloudy and faintly coloured specimens ; the usual tints are grey, brown, yel low or white ; and as rarities, red, green, blue and black stones have been found. The col our can sometimes be removed or changed at a high temperature, but generally returns on cooling. It is therefore more probably due to metallic oxides than to hydrocarbons.

Sir William Crookes has, however, changed a pale yellow diamond to a bluish-green colour by keeping it embedded in radium bromide for II weeks. Diamond may break with a conchoidal frac ture, but the crystals always cleave readily along planes parallel to the octahedron faces; of this property the diamond cutters avail themselves when reducing the stone to the most convenient form for cutting; a sawing process, however, has now been intro duced, which is preferable to that of cleavage. It is the hardest known substance (though tantalum, or an alloy of tantalum, now competes with it) and is chosen as ten in the mineralogist's scale of hardness; the Borneo stones are said to be harder than others. The specific gravity ranges from 3.56 to 3.5o, generally about The co-efficient of expansion increases very rapidly above and diminishes very rapidly at low temperatures; the maxi mum density is attained when a temperature of about —42° Centigrade is reached.

Brilliancy and "Fire..

The very high refractive power (in dex= 2.417 for sodium light) gives the stone its extraordinary brilliancy ; for light incident within a diamond at a greater angle than 241° is reflected back into the stone instead of passing through it ; the corresponding angle for glass is 404 ° . The very high dispersion (index for red light=2.4o2, for blue light=2.46o) gives it the wonderful "fire"—or display of spectral colours. Un like other cubic crystals, diamond experiences a diminution of re fractive index with increase of temperature. It is very transpar ent for Rontgen rays, whereas paste imitations are opaque. It is a good conductor of heat, and therefore feels colder to the touch than glass and imitation stones. The diamond has also a some what greasy feel. The specific heat increases rapidly with rising temperature up to 6o° C., and then more slowly. Crystals be longing to the cubic system should not be birefringent unless strained; diamond often displays double refraction particularly in the neighbourhood of inclusions, both liquid and solid ; this is probably due to strain, and the spontaneous explosion of diamonds has often been observed. Diamond differs from graphite in being a bad conductor of electricity; it becomes positively electrified by friction. The electrical resistance is about that of ordinary glass, and is diminished by one-half during exposure to Rontgen rays; the dielectric constant (16) is greater than that which should cor respond to the specific gravity.

The phosphorescence produced by friction has been known since the time of Robert Boyle (1663) ; the diamond becomes luminous in a dark room after exposure to sunlight or in the presence of radium; and many stones phosphoresce beautifully (generally with a pale green light) when subjected to the electric discharge in a vacuum tube. Some diamonds are more phosphorescent than others, and different faces of a crystal may display different tints. The combustibility of the diamond was predicted by Sir Isaac Newton on account of its high refractive power; it was first established experimentally by the Florentine academicians in i 694. In oxygen or air diamond burns at about 85o°, and only continues to do so if maintained at a high temperature ; but in the absence of oxidizing agents it may be raised to a much higher tempera ture. It is, however, infusible at the temperature of the electric arc, but becomes blackened superficially. Experiments on the combustion of diamond were made by Smithson Tennant (1797) and Sir Humphry Davy (1816), with the object of proving that it is pure carbon. Diamond is insoluble in acid and alkalis, but is oxidized on heating with potassium bichromate and sulphuric acid.

Diamond

Uses of the Diamond.—The use of the diamond for other pur poses than jewellery depends upon its extreme hardness : it has always been the only material used for cutting or engraving the diamond itself. The employment of powdered bort (q.v.) and the lapidary's wheel for faceting diamonds was introduced by L. von Berquen of Bruges in 1476. Diamonds are now employed not only for faceting precious stones, but also for cutting and drilling glass, porcelain, etc. ; for fine engraving such as scales; in dentis try for drilling ; as a turning tool for electric-light carbons, hard rubber, etc.; and occasionally for finishing accurate turning work. It is also used for bearings in watches and electric meters. The best glaziers' diamonds are chosen from crystals such that a natu ral curved edge can be used. For rock drills, and revolving saws for stone cutting, either diamond, bort or carbonado (q.v.) is employed, set in steel tubes, discs or bands. Rock drilling is the most important industrial application ; and for this, owing to its freedom from cleavage, the carbonado is more highly prized than diamond. Another application of the diamond is for wire drawing; a hole tapering towards the centre is drilled through a diamond, and the metal is drawn through this. No other tool is so durable, or gives such uniform thickness of wire.

Distribution and Mining.—The most important localities for diamonds have been : (I) India, where they were mined from the earliest times till the close of the i9th century ; (2) South America, where they have been mined since the middle of the i 8th century; and (3) South Africa, to which almost the whole of the diamond-mining industry has been transferred since 1870.

Indian Diamonds.—The diamond is here found in ancient sandstones and conglomerates, and in the river gravels and sands derived from them. The sandstones and conglomerates belong to the Vindhyan formation and overlie the old crystalline rocks : the diamantiferous beds are well defined, often not more than if t. in thickness, and contain pebbles of quartzite, jasper, sandstone, slate, etc. The mines fall into five groups situated on the eastern side of the Deccan plateau. The mining has always been carried on by natives of low caste, and by primitive methods which do not differ much from those described by the French merchant, Jean Baptiste Tavernier (160 s-89) , who paid a prolonged visit to most of the mines between 1638 and 1665 as a dealer in precious stones.

At some of the Indian localities spasmodic mining has been car ried on at different periods for centuries ; at some the work which had been long abandoned was revived in recent times, at others it has long been abandoned altogether. Many of the large stones of antiquity were probably found in the Kollar group, where Tav ernier found 6o,000 workers in 1645 (?) , the mines, according to native accounts, having been discovered about ioo years previ ously. Golconda was the fortress and the market for the diamond industry at this group of mines, and so gave its name to them. Very few Indian diamonds now find their way out of the country, and so far as the world's supply is concerned, Indian mining of dia monds may be considered extinct. The first blow to this industry was the discovery of the Brazilian mines in Minas Geraes and Bahia.

Brazilian Diamonds.—Diamonds were found about 17 2 5 at Tejuco (now Diamantina) in Minas Geraes, and the mining be came important about 1740. The Rio Abaete district was worked on a considerable scale between 1785 and 1807, but is now aban doned. Diamantina is at present the most important district ; it occupies a mountainous plateau, and the diamonds are found both on the plateau and in the river valleys below it. The mountains consist here of an ancient laminated micaceous quartzite, which is in parts a flexible sandstone known as itacolumite, and in parts a conglomerate. The diamond is found under three con ditions: (1) in the gravels of the present rivers, embedded in a ferruginous clay-cemented conglomerate known as cascalho; (2) in terraces occupying higher levels; (3) in plateau deposits em bedded in the red clay which cements the larger blocks. The ter races are probably a first concentration of the plateau material by the old rivers ; and the cascalho a second concentration by the modern rivers.

In recent years the Minas Geraes mines have been rivalled by the yield in Bahia. The diamond here occurs in river gravels and sands associated with the same minerals as in Minas Geraes; since 1844 the richest mines have been worked in the Serra de Cincora, where the mountains are intersected by the River Para guassu and its tributaries; it is said that there were as many as 20,000 miners working here in 1845, and it was estimated that carats were produced in Bahia in 1858. But the enormous development of the South African mines, which in 1906 supplied about 90 % of the world's produce, has thrown the Brazilian pro duction into the shade.

African Diamonds.—The first discovery was made in 1867 by Dr. W. G. Atherstone, who identified as diamond a pebble ob tained from a child in a farm on the banks of the Orange river and brought by a trader to Grahamstown; it was bought for f soo and displayed in the Paris Exhibition of that year. In 1869 a stone weighing 831 carats was found near the Orange river; this was purchased by the earl of Dudley for f 2 s,000 and became famous as the "Star of South Africa." A rush of prospectors at once took place to the banks of the Orange and Vaal rivers, and resulted in considerable discoveries, so that in 187o there was a mining camp of no less than io,000 persons on the "River Dig gings." In the River Diggings the mining was carried on in the coarse river gravels, and by the methods of the Brazilian negroes and of gold placer-miners. A diggers' committee limited the size of claims to 3of t. square, with free access to the river bank; the gravel and sand were washed in cradles provided with screens of perforated metal, and the concentrates were sorted by hand on tables by means of an iron scraper.

But towards the close of 187o stones were found at Jagersfon tein and at Dutoitspan, far from the Vaal river, and led to a second great rush of prospectors, especially to Dutoitspan, and in 1871 to what is now the Kimberley mine in the neighbourhood of the latter. At each of these spots the diamantiferous area was a roughly circular patch of considerable size, and in some it occu pied the position of one of those depressions or "pans" so fre quent in South Africa. These "dry diggings" were therefore at first supposed to be alluvial in origin like the river gravels; but it was soon discovered that, below the red surface soil and the underlying calcareous deposit, diamonds were also found in a layer of yellowish clay about Soft. thick known as "yellow ground." Below this again was a hard bluish-green serpentinous rock which was at first supposed to be barren bed-rock; but this also contained the precious stone, and has become famous, under the name of "blue ground," as the matrix of the South African diamonds. The yellow ground is merely decomposed blue ground. In the Kimberley district five of these round patches of blue ground were found within an area little more than 3m. in diameter; that at Kimberley occupying ten ac., that at Dutoitspan 23 acres. There were soon 50,00o workers on this field, the canvas camp was replaced by a town of brick and iron surrounded by the wooden huts of the natives, and Kimber ley became an important centre.

It was soon found that each mine was in reality a huge vertical funnel or crater descending to an unknown depth, and filled with diamantiferous blue ground. At first each claim was an independ ent pit 31 f t. square sunk into the blue ground; the diamantiferous rock was hoisted by bucket and windlass, and roadways were left across the pit to provide access to the claims. But the roadways soon fell in, and ultimately haulage from the claims could only be provided by means of a vast system of wire ropes extending from a triple staging of windlasses erected round the entire edge of the mine, which had by this time become a huge open pit ; the ropes from the upper windlasses extended to the centre, and those from the lower tier to the sides of the pit ; covering the whole mass like a gigantic cobweb of taut steel ropes. The buckets of blue ground were hauled up these ropes by means of horse whims, and in steam winding engines began to be employed. By this time also improved methods in the treatment of the blue ground were introduced. It was carried off in carts to open spaces, where an exposure of some weeks to the air was found to pulverize the hard rock far more efficiently than the old method of crushing with mallets. The placer-miner's cradle and rocking-trough were replaced by puddling troughs stirred by a revolving comb worked by horse power ; reservoirs were constructed for the scanty water supply, bucket elevators were introduced to carry away the tail ings; and the natives were confined in compounds. For these improvements co-operation was necessary; the better claims, which in 1872 had risen from LI oo to more than 14,000 in value, began to be consolidated, and a mining board was introduced. In a very few years, however, the open pit mining was rendered impossible by the mud rushes, by the falls of the masses of barren rock known as "reef," which were left standing in the mine, and by landslips from the sides, so that in 1883, when the pit had reached a depth of about 400f t., mining in the Kimberley crater had be come almost impossible. By 1889, in the whole group of mines, Kimberley, Dutoitspan, De Beers and Bultfontein, open pit work ing was practically abandoned. Meanwhile mining below the bot tom of the pits by means of shafts and underground tunnels had been commenced; but the full development of modern methods dates from the year 1889 when Cecil Rhodes and Alfred Beit, who had already secured control of the De Beers mine, acquired also the control of the Kimberley mine, and shortly afterwards consolidated the entire group in the hands of the De Beers Com pany (see KIMBERLEY).

The scene of native mining was now transferred from the open pit to underground tunnels; the vast network of wire ropes disappeared, and with it the cosmopolitan crowd of busy miners working like ants at the bottom of the pit. In place of all this, the visitor to Kimberley encounters at the edge of the town only a huge crater, silent and apparently deserted, with no visible sign of the great mining operations which are conducted far below the surface.

A vertical shaft through the basalt, shale and granite is sunk in the vicinity of the mine, and from this horizontal tunnels are driven into the pipe at different levels separated by intervals of 40 feet. Through the blue ground itself on each level a series of parallel tunnels about i2oft. apart are driven to the opposite side of the pipe, and at right angles to these, and 36ft. apart, another series of tunnels. When the tunnels reach the side of the mine they are opened upwards and sideways so as to form a large cham ber, and the overlying mass of blue ground and debris is allowed to settle down and fill up the gallery. On each level this process is carried somewhat farther back than on the level below (fig. 7) ; material is thus continually withdrawn from one side of the mine and extracted by means of the rock shaft on the opposite side, while the superincumbent debris is continually sinking, and is allowed to fall deeper on the side farthest from the shaft as the blue ground is withdrawn from beneath it. In i9o5 the main shaft had been sunk to a depth of 2,600ft. at the Kimberley mine.

For the extraction and treatment of the blue ground the De Beers Company in its great winding and washing plant employs labour-saving machinery on a gigantic scale. The ground is trans ferred in trucks to the shaft where it is automatically tipped into skips holding 96 cu. ft. (six truck loads) ; these are rapidly hoisted to the surface, where their contents are automatically dumped into side-tipping trucks, and these in turn are drawn away in a con tinual procession by an endless wire rope along the tram lines leading to the vast "distributing floors." These are open tracts upon which the blue ground is spread out and left exposed to sun and rain until it crumbles and disintegrates, the process being hastened by harrowing with steam ploughs; this may require a period of three or six months, or even a year. The stock of blue ground on the floors at one time in 19o5 was nearly 4,5oo,000 loads. The disintegrated ground is then brought back in the trucks and fed through perforated cylinders into the washing pans; the "hard blue" ground which has resisted disintegration on the floors, and the lumps which are too big to pass the cylindrical sieves, are crushed before going to the pans. These are shallow cylindrical troughs containing muddy water in which the diamonds and other heavy minerals (concentrates) are swept to the rim by revolving toothed arms, while the lighter stuff escapes near the centre of the pan. The concentrates are then passed over sloping tables (pulsator) and shaken to and fro under a stream of water which effects a second concentration of the heaviest material.

Until recently the final separation of the diamond from the concentrates was made by hand picking, but even this has now been replaced by machinery, owing to the remarkable discovery that a greased surface will hold a diamond while allowing the other heavy minerals to pass over it. The concentrates are washed down a sloping table of corrugated iron which is smeared with grease, and it is found that practically all the diamonds adhere to the table, and the other minerals are washed away. At the large and important Premier mine in the Transvaal the Elmore process, used in British Columbia and in Wales for the separation of metallic ores, has been also introduced. In the Elmore process oil is employed to float off the materials which adhere to it, while the other materials remain in the water, the oil being separated from the water by centrifugal action.

In all the South African mines the diamonds are not only crystals of various weights from fractions of a carat to iso carats, but also occur as microscopic crystals disseminated through the blue ground. In spite of this, however, the average yield in the profitable mines is only from 0.2 carat to 0-6 carat per load of 1,600 lb., or on an average about grs. per ton. The annual out put of diamonds from the De Beers mines was valued in 1906 at nearly is,000,000, the value per carat ranging from about 35 to 7o shillings.

Pipes similar to those which surround the Kimberley have been found in other parts of South Africa. One of the best known is that of Jagersfontein, which was really the first of the dry diggings (discovered in 187o). This large mine is near Fauresmith and 8om. to the south of Kimberley. In 19o5 the year's production from the Orange River Colony mines was more than 32o,000 carats, valued at 1938,000. But by far the largest of all the pipes hitherto discovered is the Premier mine in the Transvaal, about 3oom. to the east of Kimberley. This was -discovered in 1902 and occupies an area of about 75 acres. Comparatively few of the pipes which have been discovered are at all rich in diamonds, and many are quite barren; some are filled with "hard blue" which, even if diamantiferous, may be too expensive to work.

The most competent South African geologists believe all these remarkable pipes to be connected with volcanic outbursts which occurred over the whole of South Africa during the Cretaceous period (after the deposition of the Stormberg beds), and drilled these enormous craters through all the later formations. Over a great part of Cape Colony have been discovered what are prob ably similar pipes filled with agglomerates, breccias and tuffs, and some with basic lavas.

The River Diggings on the Vaal river are still worked upon a small scale, but the production from this source is so limited that they are of little account in comparison with the mines in the blue ground. The stones, however, are good; since they differ some what from the Kimberley crystals it is probable that they were not derived from the present pipes. Considerable finds of diamonds were reported in 1905 and 1906 from gravels at Somabula near Gwelo in Rhodesia. Diamonds have also been reported from kimberlite "pipes" in Rhodesia.

The South African output in 1926 was 3,000,00o carats. Dia monds have been found in considerable numbers in sand near Luderitz bay in South-West Africa (1908) ; the output from this district in 1926 was 515,00n carats. The Congo has become an important source since the first discovery in 1903 ; the diamonds are found in alluvial deposits of the River Kasai and its affluents. The output in 1926 was 1,108,000 carats from Belgian Congo, and I 50,00o carats from Angola. Other African localities are the Gold Coast and Tanganyika.

Other Localities.

In addition to the South American localities mentioned above, small diamonds have also been mined since their discovery in 1890 on the River Mazaruni in British Guiana, and finds have been reported in the gold washings of Dutch Guiana. The output from British Guiana in 1926 was 182,00o carats. Borneo has possessed a diamond industry since the island was first settled by the Malays. Australia has yielded diamonds in alluvial deposits near Bathurst (where the first discovery was made in 1851) and at other places in New South Wales; in South Australia; in Victoria; in Western Australia; and in Queensland. In Tasmania also diamonds have been found in the Corinna gold fields. Europe has produced few diamonds. They have been found (1829) in the gold washings of Bissersk, and at other spots in the Urals. Also in Lapland and Siberia. In North America small stones have been found in alluvial deposits, mostly aurifer ous, in Georgia, North and South Carolina, Kentucky, Virginia, Tennessee, Wisconsin, California, Oregon and Indiana. Con siderable interest attaches to the diamonds found in Wisconsin, Michigan and Ohio near the Great Lakes, for they are here found in the terminal moraines of the great glacial sheet which is sup posed to have spread southwards from the region of Hudson bay.

Origin of the Diamond in Nature.--It

appears from the foregoing account that at most localities the diamond is found in alluvial deposits probably far from the place where it originated. The minerals associated with it do not afford much clue to the original conditions; they are mostly heavy minerals derived from the neighbouring rocks, in which the diamond itself has not been observed.

There are only a few localities at which the diamond has been supposed to occur in its original matrix—in India, in Minas Geraes, and at Inverell in New South Wales, but the evidence is certainly not sufficient to establish the presence of an original matrix. Finally there is the remarkable occurrence in the blue ground of the African pipes.

There has been much controversy concerning the nature and origin of the blue ground itself ; and even granted that (as is gen erally believed) the blue ground is a much serpentinized volcanic breccia consisting originally of an olivine-bronzite-biotite rock (the so-called kimberlite), it contains so many rounded and angular fragments of various rocks and minerals that it is difficult to say which of them may have belonged to the original rock, and whether any were formed in situ, or were brought up from below as in clusions. Carvill Lewis believed the blue ground to be true eruptive rock, and the carbon to have been derived from the bituminous shales of which it contains fragments. The Kimberley shales, which are penetrated by the De Beers group of pipes, were, however, certainly not the source of the carbon at the Premier (Transvaal) mine, for at this locality the shales do not exist. The view that the diamond may have crystallized out from solution in its present matrix receives some support from the experiments of W. Luzi, J. Friedlander, R. von Hasslinger and J. Wolff. E. Cohen, who regarded the pipes as of the nature of a mud volcano, and the blue ground as a kimberlite breccia altered by hydro thermal action, thought that the diamond and accompanying minerals had been brought up from deep-seated crystalline schists. Other authors have sought the origin of the diamond in the action of the hydrated magnesian silicates on hydrocarbons derived from bituminous schists, or in the decomposition of metallic carbides.

Of great scientific interest in this connection is the discovery of small diamonds in certain meteorites, both stones and irons; e.g., in the stone which fell at Novo-Urei in Penza, Russia, in 1886, in a stone found at Carcote in Chile, and in the iron found at Canon Diablo in Arizona. Graphitic carbon in cubic form (cliftonite) has also been found in certain meteoric "irons," and is now gen erally believed to be altered diamond. The claim by H. Moissan to have produced the diamond artificially, by allowing dissolved carbon to crystallize out at a high temperature and pressure from molten iron, coupled with the occurrence in meteoric iron, has led Sir William Crookes and others to conclude that the mineral may have been derived from deep-seated iron containing carbon in solution (see the article GEM, ARTIFICIAL) .

On the other hand, the occurrence in meteoric stones, and the experiments mentioned above, show that the diamond may also crystallize from a basic magma, capable of yielding some of the metallic oxides and ferro-magnesian silicates ; a magma, therefore, which is not devoid of oxygen. This is still more forcibly sug gested by the remarkable eclogite boulder found in the blue ground of the Newlands mine, not far from the Vaal river, and described by T. G. Bonney. The boulder is a crystalline rock, and is studded with diamond crystals; a portion of it is preserved in the British Museum (Natural History). Similar boulders have also been found in the blue ground elsewhere. It seems therefore that a holo-crystalline pyroxene-garnet rock may be one source of the diamond found in blue ground. Some regard the eclogite boulders as derived from deep-seated crystalline rocks, others as concretions in the blue ground. None of the inclusions in the diamond gives any clue to its origin.

Finally, then, both experiment and the natural occurrence in rocks and meteorites suggest that diamond may crystallize not only from iron but also from a basic silicate magma, possibly from various rocks consisting of basic silicates. The blue ground of South Africa may be the result of the serpentinization of several such rocks, and although now both brecciated and serpentinized some of these may have been the original matrix. A circumstance often mentioned in support of this view is the fact that the diamonds in one pipe generally differ somewhat in character from those of another, even though they be near neighbours.

History of Diamonds.

All the famous diamonds of antiquity must have been Indian stones. The first author who described the Indian mines at all fully was the Portuguese, Garcia de Orta (1565), who was physician to the viceroy of Goa. Before that time there were only legendary accounts like that of Sindbad's "Valley of the Diamonds," or the tale of the stones found in the brains of serpents. V. Ball thinks that the former legend orig inated in the Indian practice of sacrificing cattle to the evil spirits when a new mine is opened; birds of prey would naturally carry off the flesh, and might give rise to the tale of the eagles carrying diamonds adhering to the meat.

The following are some of the famous diamonds of the world:- A large stone found in the Golconda mines and said to have weighed 787 carats in the rough, before being cut by a Venetian lapidary, was seen in the treasury of Aurangzeb in 1665 by Taver nier, who estimated its weight after cutting as 28o ( ?) carats, and described it as a rounded rose-cut-stone, tall on one side. The name Great Mogul has been frequently applied to this stone. Tavernier states that it was the famous stone given to Shah Jahan by the emir Jumla. The Orloff, stolen by a French soldier from the eye of an idol in a Brahmin temple, stolen again from him by a ship's captain, was bought by Prince Orloff for f 90,000, and given to the empress Catharine II. It weighs 1944 carats, is of a somewhat yellow tinge, and is among the Russian Crown jewels. The Koh-i-nor, which was in 1739 in the possession of Nadir Shah, the Persian conqueror, and in 1813 in that of the raja of Lahore, passed into the hands of the East India Company and was by them presented to Queen Victoria in 185o. It then weighed carats, but was recut in London by Amsterdam workmen, and now weighs Io6 carats. There has been much discussion concerning the possibility of this stone and the Orloff being both fragments of the Great Mogul. The Mogul Saber in his memoirs (1526) relates how in his conquest of India he captured at Agra the great stone weighing 8 mishkals, or 32o ratis, which may be equivalent to about 187 carats. The Koh-i-nor has been identified by some authors with this stone and by others with the stone seen by Tavernier. Tavernier, however, subsequently described and sketched the diamond which he saw as shaped like a bisected egg, quite different therefore from the Koh-i-nor. Nevil Story Mas kelyne has shown reason for believing that the stone which Taver nier saw was really the Koh-i-nor, and that it is identical with the great diamond of Baber; and that the 280 carats of Tavernier is a misinterpretation on his part of the Indian weights. He suggests that the other and larger diamond of antiquity which was given to Shah Jahan may be one which is now in the treasury of Teheran, and that this is the true Great Mogul which was confused by Tavernier with the one he saw. (See Ball, Appendix I. to Taver nier's Travels (1889) ; and Maskelyne, Nature, 44, P. 555.) The Regent or Pitt diamond is a magnificent stone found in either India or Borneo; it weighed 410 carats and was bought for £2o,400 by Pitt, the governor of Madras; it was subsequently, in 1717, bought for L8o,000 (or, according to some authorities, £135,000) by the duke of Orleans, regent of France; it was re duced by cutting to 136i s carats; was stolen with the other Crown jewels during the Revolution, but was recovered and is still in France. The Akbar Shah was originally a stone of 116 carats with Arabic inscriptions engraved upon it ; after being cut down to 71 carats it was bought by the gaikwar of Baroda for £35,000. The Nizam, now in the possession of the nizam of Hyderabad, is sup posed to weigh 277 carats; but it is only a portion of a stone which is said to have weighed 44o carats before it was broken. The Sancy, weighing 53H carats, is said to have been successively the property of Charles the Bold, de Sancy, Queen Elizabeth, Hen rietta Maria, Cardinal Mazarin, Louis XIV. ; to have been stolen with the Pitt during the French Revolution; and subsequently to have been the property of the king of Spain, Prince Demidoff and an Indian prince.

The Great Table, a rectangular stone seen by Tavernier in 1642 at Golconda, was found by him to weigh carats ; Maskelyne regards it as identical with the Darya-i-nur, which is also a rectangular stone weighing about 186 carats in the possession of the shah of Persia. Another stone, the Taj-e-mah, belonging to the shah, is a pale rose pear-shaped stone and is said to weigh 146 carats.

Coloured Indian diamonds of large size are rare; the most famous are: a beautiful blue brilliant, 67A. carats, cut from a stone carats brought to Europe by Tavernier. It was stolen from the French Crown jewels with the Regent and was never recovered. The Hope, 44+ carats, has the same colour and is probably a portion of the missing stone : it was so-called as forming part of the collection of H. T. Hope bought for £i8, 000, and was sold again in 1906 (resold 1909). Two other blue diamonds are known, weighing 13 and 14 carats, which may also be portions of the French diamond. The Dresden Green, one of the Saxon Crown jewels, 4o carats, has a fine apple-green colour. The Florentine, I33$ carats, one of the Austrian Crown jewels, is a very pale yellow.

The most famous Brazilian stone was the Star of the South, found in 1853, when it weighed 2 542 carats and was sold for £40,000; when cut it weighed 125 carats and was bought by the gaikwar of Baroda for £8o,000.

The Largest Diamond.

Many large stones have been found in South Africa; some are yellow but some are as colourless as the best Indian or Brazilian stones. The most famous are the fol lowing:—The Star of South Africa, or Dudley, mentioned above, 832 carats rough, 464 carats cut. The Stewart, 288i carats rough, 120 carats cut. Both these were found in the river dig gings. The Porter Rhodes from Kimberley, of the finest water, weighed about 15o carats. The Victoria, 18o carats, was cut from an octahedron weighing 4572 carats, and was sold to the nizam of Hyderabad for £400,000. The Tiffany, a magnificent orange-yellow stone, weighs 12s2 carats cut. A yellowish octa hedron found at De Beers weighed 4284 carats, and yielded a brilliant of 2282 carats. Some of the finest and largest stones have come from the Jagersfontein mine ; one, the Jubilee, found in 1895, weighed 634 carats in the rough and 239 carats when cut. Until 19o5 the largest known diamond in the world was the Ex celsior, found in 1893 at Jagersfontein by a native while loading a truck. It weighed 9692 carats, and was ultimately cut into ten stones weighing from 68 to 13 carats. But all previous records were surpassed in 1905 by the Cullinan Diamond more than three times the size of any known stone, which was found in the yellow ground at the newly discovered Premier mine in the Transvaal. It was purchased by the Transvaal Government in 1907 and pre sented to King Edward VII. It was sent to Amsterdam to be cut, and in 1908 was divided into nine large stones, the four largest weighing 5162 carats, 3o9 g carats, 92 carats and 62 carats re spectively, and a number of small brilliants. The Jonker diamond, weighing 726 carats, was found in Elandsfontein in 1934, and was sold in May, 1935, to an American for about £15o,000.

Diamonds are invariably weighed in carats. One English carat =3.17 grains=•2,o53 grams. One metric carat (now nearly uni versally used)=•2oo grams or 200 milligrams. (See CARAT.) BIBLIOGRAPHY.-Boetius de Boot, Gemmarum et lapidum historia Bibliography.-Boetius de Boot, Gemmarum et lapidum historia (1609) ; D. Jeffries, A Treatise on Diamonds and Pearls (i757) ; J. Mawe, Travels in the Interior of Brazil (1812) ; Treatise on Diamonds and Precious Stones (1813) ; M. Pinder, De adamante (1829) ; J. Murray, Memoir on the Nature of the Diamond (1831) ; C. Zerenner, De adamante dissertatio (185o) ; H. Emanuel, Diamonds and Precious Stones (1865) ; A. Schrauf, Edelsteinkunde (1869) ; N. Jacobs and N. Chatrian, Monographie du diamant (188o) ; V. Ball, Geology of India (1881) ; C. W. King, The Natural History of Precious Stones and Precious Metals (1883) ; M. E. Boutan, Le Diamant (1886) ; S. M. Burnham, Precious Stones in Nature, Art and Literature (1887) ; P. Groth, Grundriss der Edelsteinkunde (1887) ; A. Liversidge, The Minerals of New South Wales (1888) ; Tavernier's Travels in India, trans. by V. Ball (1889) ; E. W. Streeter, The Great Diamonds of the World (1896) ; H. C. Lewis, The Genesis and Matrix of the Diamond (1897) ; L. de Launay, Les Diamants du Cap (1897) ; C. Hintze, Handbuch der Mineralogie (1898) ; E. W. Streeter, Precious Stones and Gems (6th ed., 1898) ; J. D. Dana, System of Mineralogy (1899) ; G. F. Kunz and others, The Produc tion of Precious Stones (in annual, Mineral Resources of the United States) ; M. Bauer, Precious Stones (trans. L. J. Spencer, 1904) ; A. W. Rogers, An Introduction to the Geology of Cape Colony (19o5) ; Gardner F. Williams, The Diamond Mines of South Africa (rev. ed., 1906) ; G. F. Kunz, "Diamonds, a study of their occurrence in the United States, with descriptions and comparisons of those from all known localities" (U.S. Geol. Survey, 1909) ; P. A. Wagner, Die Diamant f iihrenden Gesteine Siida f rikas (1909) and The Diamond Fields of S. Africa (1914) ; A. v. Fersmann and V. Goldschmidt, Der Diamant (1911) . (H. A. M.)

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