DIAMOND, a genus of earths of the siliceous kind, called Adamas Gemma by the Latins, Demant by the Germans and Swedes, and Diamant by the French, is the hardest of all stones hitherto discovered; commonly clear or transparent; though this property may perhaps belong only to the crystals, and not to the rock from which they originate. When brought to Europe in its rough state, it is either in the form of roundish pebbles, with shining surfaces, or of octædral crystals; but though they generally appear in octædral forms, yet their crystals are frequently irregular, especially when the surface inclines to crystallize during the shooting of the whole crystal, and also when several of them unite in one group; in which case the one hinders the other from assuming a regular form. Mr Magellan, however, informs us, that diamonds sometimes assume other forms. He has seen a rough diamond in its native state, of a regular cubical form, with its angles truncated or cut off; likewise another belonging to Dr Combe of London, whose square sides were naturally joined by two very narrow long facets, forming angles of about 120 degrees; and the corners were quite perfect.
Though the diamond is commonly clear and pellucid, yet some of them are met with of a rose colour, or inclining to green, blue, or black, and some have black specks. Tavernier saw one in the treasury of the Mogul, with black specks in it, weighing about 56 carats; and he informs us, that yellow and black diamonds are produced in the mines at Carnatica. Mr Dutens also relates, that he saw a black diamond at Vienna in the collection of the prince de Lichtenstein. Some diamonds have a greenish crust; and of these M. Tavernier relates, that they burst into pieces while working into a proper shape, or in the very act of polishing on the wheel. In confirmation of this, he mentions a large diamond worth upwards of 5000 l. Sterling, which burst into nine pieces while polishing on the wheel at Venice.
The finest diamonds are those of a complexion like that of a drop of pure water. It is likewise a valuable property if they are of a regular form and truly made; as also that they be free from stains, spots, specks, flaws, and cross veins. If diamonds are tinctured yellow, blue, green, or red, in a high degree, they are next in esteem; but if they are tinctured with these colours only in a low degree, the value of them is greatly diminished. There are also diamonds of other complexions; such as brown, and those of a dark hue: the first resembling the brownish sugar-candy, and the latter dusky iron. In the Philosophical Commerce of Arts, Dr Lewis tells us of a black diamond that he himself had seen. At a distance, it looked uniformly
VOL. VI. Part I.
black; but on closer examination appeared in some parts transparent, and in others charged with foulness, on which the black hue depended.
These gems are lamellated, consisting of very thin plates like those of tale, but very closely united; the direction of which must be found out by lapidaries before they can work them properly: Such as have their foliated substance not in a flat position, are called by the workmen diamonds of nature.
The names of oriental and occidental, given by jewelers to this and all other precious stones, have a different meaning from the obvious sense; the finest and hardest being always called oriental, whether they be produced in the east or not. Those called occidental are of inferior value; but according to Mr Jeffries, who has written a treatise on the subject, the diamonds of Brasil equal the finest oriental ones. The art of cutting these gems was invented in 1476 by Louis de Berquen a native of Bruges in the Austrian Netherlands. This stone becomes luminous in the dark, by exposure during a certain time to the rays of the sun; by heating it in a crucible; by plunging it in boiling water; or by rubbing it with a piece of glass. By friction it acquires an electrical property, by which it attracts the substance used for foils called black massic, and other light matters. The author of the Chemical Dictionary says, that diamonds are refractory in the fire, and even apyrous. Nevertheless, experiments have been made, which prove that diamonds are capable of being dissipated, not only by the collected heat of the sun, but also by the heat of a furnace. Mr Boyle says, that he perceived certain acrid and penetrating exhalations from diamonds exposed to fire. A diamond by exposure to a concave speculum, the diameter of which was 40 inches, was reduced to an eighth part of its weight. In the Giornale de Letterati d' Italia, tom. viii. art. 9. we may read a relation of experiments made on precious stones, by order of the grand duke of Tuscany, with a burning lens, the diameter of which was two thirds of a Florentine ell, near the focus of which was placed another smaller lens. By these experiments we find, that diamonds were more altered by solar heat than most of the other precious stones, although not the least appearance of a commencing fusion was observable. A diamond weighing 30 grains, thus exposed during 30 seconds, lost its colour, lustre, and transparency, and became of an opaque white. In five minutes, bubbles appeared on its surface; soon afterwards it burst into pieces, which were dissipated; and the small fragment which remained was capable of being crushed into fine powder by the pressure of the blade of a knife. Neither the addition of glass, flints, sulphur, metals, or salt of tartar, prevented this dissipation.
Diamond. pation of diamonds, or occasioned any degree of fusion. By this heat rubies were softened, and lost some of their colour, but preserved their form and weight. By addition of a third lens, a further degree of fusion was given to rubies. Even then rubies could not be made to unite with glass. By having been exposed to this heat, the surface of the rubies which had suffered fusion, lost much of their original hardness, and were nearly as soft as crystal. But their internal parts, which had not been fused, retained their hardness. Emeralds by this heat were rendered white, or of various colours, and soon afterwards were fused. They were found to have lost part of their weight, and to be rendered less hard and brittle.
Experiments were also made by order of the emperor Francis I. on precious stones; from which we find, that diamonds were entirely dissipated by having been exposed in crucibles to a violent fire of a furnace during 24 hours; while rubies by the same heat were not altered in weight, colour, or polish. By exposing diamonds during two hours only at a time, the following alterations produced on them by fire were observed. First, they lost their polish; then they were split into thin plates; and, lastly, totally dissipated. By the same fire, emeralds were fused. See Magazin de Hambourg, tom. xviii.
The action of fire on diamonds has, notwithstanding the above mentioned experiments, been lately doubted in France; and the question has been agitated by several eminent chemists with much interest, and numerous experiments have been made which throw some light on the subject. M. D'Arcet found, not only that diamonds included in porcelain crucibles close, or covered with perforated lids, and exposed to the long and intense heat of a porcelain furnace, were perfectly dissipated; but also, that these stones could in a few hours be totally volatilised with a much inferior degree of heat, by exposing them in a cappel, under the muffle of an assay-furnace. In this latter experiment, he observed that the dissipation was gradual, and that it was effected by a kind of exfoliation. The dissipation of diamonds exposed in cappels was confirmed by M. Macquer; who further observed, that the diamonds were, before the dissipation began, rendered, by the fire, brilliant and shining, as it were, with a phosphoric light. In order to determine whether the dissipation of diamonds was actually effected by their reduction into vapour, or by a combustion or other effect of air upon them, Messrs Lavoisier, Macquer, and Cadet, exposed diamonds to intense heat in an earthen retort, during several hours, but without any other effect than that their polish was destroyed, and about 4th of their weight diminished. M. Mitouard put diamonds in a tobacco-pipe filled with pounded charcoal and accurately closed with lute. He further secured the diamonds from access of air or flame, by placing the tobacco-pipe in a crucible, to which another crucible was inverted and carefully luted. The diamonds, thus secluded from external air, having been exposed to the most intense heat which could be excited in a well constructed furnace, were not thereby altered or diminished. M. Mitouard was induced to believe, that the charcoal conduced to the preservation of diamonds not merely by excluding the air, but by some peculiar property, which he supposes may be the same as that by
which this substance defends metals from destruction by fire. He was confirmed in his opinion, by observing that diamonds were not preserved from the action of fire by surrounding them with powder of chalk and of calcined hartshorn, and including them in close vessels, so well as when the charcoal had been employed. Some chemists even thought that the perfect exclusion of air alone was sufficient to preserve diamonds, and doubted whether the balls and crucibles of porcelain employed by M. D'Arcet had excluded the air with sufficient accuracy. Indeed, in one of M. D'Arcet's own experiments, a diamond included in a ball of porcelain had resisted the action of fire. In order to ascertain this question, M. Cadet exposed diamonds in covered and luted crucibles to the violent heat of a forge during two hours; by which operation the diamonds lost only th part of their weight. He infers, that the destruction of diamonds by fire in open vessels is not a true volatilization; but merely an exfoliation, caused by the fire expanding the air contained between the thin plates of which these stones consist, and that by this exfoliation or decrepitation these plates are reduced to so fine a powder as to escape observation. M. D'Arcet objected against the experiments of his adversaries, that they were not of sufficient duration to decide against his, which had lasted several days. He renewed and multiplied his experiments, which confirmed him in his opinion of the volatilization of diamonds in vessels perfectly closed; and that this effect of fire on diamonds is not a mere exfoliation or mechanical separation of the plates of which these stones consist, he infers from the parts of the diamonds pervading the most solid porcelain crucibles without being perceptible, and from the luminous appearance first noticed by M. Macquer, and which was afterwards observed by M. Roux to be an actual flame.
Diamonds are found only in the East Indies, and in Brasil in South America. The diamond mines are found only in the kingdoms of Golconda, Visapour, Bengal, and the island of Borneo. There are four mines, or rather two mines and two rivers, whence diamonds are drawn. The mines are, 1. That of Raolconda, in the province of Carnatica, five days journey from Golconda, and eight from Visapour. It has been discovered about 200 years. 2. That of Gani, or Couleur, seven days journey from Golconda eastwardly. It was discovered 140 years ago by a peasant, who digging in the ground found a natural fragment of 25 carats. 3. That of Soumelpour, a large town in the kingdom of Bengal, near the Diamond-mine. This is the most ancient of them all: it should rather be called that of Gowal, which is the name of the river, in the sand whereof these stones are found. Lastly, the fourth mine, or rather the second river, is that of Sucudan, in the island of Borneo.