Diamonds

Diamonds vary greatly in size. The Cullinan diamond, the largest ever found, which was discovered in 1905 in the yellow ground at the Premier mine, in the Transvaal, weighed 3025 3/4 carats (Carat = the seed of St. John's bread, formerly used for weights. 1 diamond carat = 3.17 grains or 0.2054 grm. 1 oz. Troy = 151.5 carats.) ( = 1⅓ lb.). Rough, small diamonds, consisting of impure crystals or fragments, are known as Bort or Boart; black diamond is Carbonado. A specimen of carbonado found in Bahia in 1895 weighed 3078 carats. The most valuable diamonds, those of the "purest water," are colourless, transparent and brilliant. One of the most beautiful of these is the Pitt or Regent diamond, weighing 136.25 carats. The celebrated Koh-i-noor, a British crown diamond, which came from India, originally weighed 186 carats, but was reduced by cutting to 106 carats. Diamonds may be cloudy or tinted grey, yellow, or brown; they may also be coloured red, green, or blue by traces of metallic oxides. The Hope diamond, which weighs 44½ carats, is blue, and is worth £25,000.

It has been discovered by Sacerdote that exposure to cathode rays deepens the original colour of the diamond. This deepening of colour appears to be permanent at ordinary temperature, but exposure to a temperature of 300°-400° C. converts the diamond to its original colour.

Diamonds have been known and valued from antiquity for their brilliance and hardness. They were first called Adamant - whence diamaunt, diamant, diamond - in a.d. 16. Pliny, a.d. 100, refers to the diamond as "the most valuable of gems, known only to kings." The gem was probably unknown to the early Hebrews, although the word is applied in Exodus xxxix, 11, to an engraved stone.

Diamonds were first discovered in the sands of India, whence they were brought to Europe in the time of the Romans. Their beauty and rarity gave rise to the superstitious belief that they could avert insanity, ward off the effect of poison, and restore domestic peace. Diamond was at one time thought to be a fossil resin or an "unctuous substance coagulated." On this account Newton believed diamond to be combustible; and the fact was first proved by the Florentine Academicians in 1694. It was observed in 1751 that diamonds disappear when heated in a furnace, while rubies remain unchanged; but Darcet and Rouelle showed in 1766 that this is not the case if the precious stones are first placed in a hermetically sealed vessel. Macquer, Cadet, and Lavoisier, about 1771, proved that diamond undergoes true combustion, burning with a flame;. and, in conjunction with Brisson and Baume, they ignited diamond, confined with air over mercury, by means of a burning- glass, and showed the formation of carbon dioxide.

Thus the nature of diamond was established; whilst its chemical identity with charcoal was proved by Smithson Tennant, who in 1797 obtained equal weights of carbon dioxide from equal weights of charcoal and diamond. Lastly, Sir Humphry Davy showed in 1814 that the carbon dioxide produced by burning diamond is pure, containing no water; and therefore that diamond contains no hydrogen.

Diamonds have been found in India, Brazil, South Africa; also in Borneo, Australia, Tasmania, North America, Guiana, and the Ural Mountains. India was the first diamond field, whence all the stones came in ancient times. The industry was centred in Golconda, which was a fortress and market for the gems, and gave its name to the adjacent mines. The discovery of diamonds in Brazil in 1725 struck a blow at the Indian industry. Tejuco, afterwards named Diamantina, in the province of Minas Geraes, is the most important Brazilian centre. It is situated on a mountainous plateau consisting of laminated micaceous rock, flexible sandstone called itacolumite, and conglomerate. The diamonds are found on the plateau and in the river gravels beneath it. Since 1844 diamonds have also been found in the neighbouring province of Bahia. In 1867 a pebble with which a child was playing on the banks of the Orange River was discovered to be a diamond, and two years afterwards a stone weighing 83½ carats, found in the same locality, was sold as the "Star of South Africa" for £25,000. "River diggings" commenced and the South African industry became established. Besides occurring in alluvial deposits, diamonds are found in South Africa in a kind of clay, called "yellow earth," fifty feet thick; and in a bluish-green rock of the nature of serpentine, known as "blue ground," which occurs beneath the "yellow earth" at Kimberley and Dutoitspan. This "blue ground" is in funnel-shaped deposits or "pipes" which are supposed to have been produced by volcanic outbursts during the cretaceous period. Formerly the earth was brought up from the open mines in buckets swung on ropes which formed a network overhead, but on account of the danger from landslips this method has been superseded by the sinking of shafts through the adjacent rock combined with tunnels. After concentration the earth is washed down a greased surface to which the diamonds adhere.