DIAMOND. 



721 





Diamond. DIAMOND, the most beautiful and valuable mine- 

 *^— v" ,i - > ral substance hitherto discovered. It is the most highly 

 prized of the gems, and, by universal consent, is placed 

 External at the head of the mineral 'kingdom. Its colours are 

 characters, various. The most frequent tints are grey and white ; 

 less frequent are the blue, red, brown, yellow, and 

 green, and the rarest of all the varieties of colour is the 

 dark brownish black. It occurs in roundish grains or 

 crystals ; and of these latter, the most frequent form is 

 the octahedron or double four-sided pyramid. Its frac- 

 ture is distinctly foliated with a fourfold cleavage, and 

 the folia of the cleavages are parallel with the planes of 

 the octahedron. Its lustre is splendent and adaman- 

 tine. It is seldom completely transparent; more ge- 

 nerally it rather inclines to semitransparent, but the 

 black variety is nearly opaque. It refracts single. It 

 is the hardest mineral hitherto discovered, hence it 

 scratches all other fossils ; and its specific gravity va- 

 ries from 3.488 to 3.600. 

 Constituent * n a history of gems, published by Boetius de Boot 

 parts. in the year 1607, it is conjectured that the diamond is 



an inflammable substance. In 1673, Boyle discovered, 

 that, when exposed to a high temperature, part of it 

 was dissipated in acrid vapours. In 1694 and 1695, 

 experiments were made in the presence of the Grand 

 Duke of Tuscany, which confirmed those of Mr Boyle, and 

 shewed that the diamond, although the hardest of mi- 

 nerals, agrees with combustible bodies, in being com- 

 bustible. In 1701, Sir Isaac Newton, in his great work 

 on optics, hinted, that from its very great refractive 

 power, it might be an unctuous substance coagulated. 

 Newton does not appear to have been acquainted with 

 the experiments made in Tuscany ; and, besides, a con-, 

 siderable part of his work on optics was written in. 1675. 

 Since that period, the diamond has been often examin- 

 ed by chemists, and they find, that, when heated to the 

 temperature of 14° of Wedgworth's pyrometer, or not 

 so high as the melting point of silver, it gradually dis- 

 sipates and burns ; and combines with nearly the same 

 quantity of oxygen, and forms the same proportion of 

 carbonic acid as charcoal. Hence it consists principally 

 of carbon.* 

 Physical When rubbed, whether rough or polished, it shews 



characters, positive electricity ; whereas quartz and the other pre- 

 cious stones, if rough, afford negative electricity, but 

 when polished, positive electricity. It becomes phos- 

 phorescent when exposed to the rays of the sun. Ma- 

 ny diamonds, however, do not become phosphorescent, 

 although agreeing in colour, form, and transparency, with 

 these which readily become luminous. The smaller 

 acquire this property by a much shorter exposure to 

 the light than the larger ones ; sometimes a diamond 

 that is not phosphorescent, by the mere action of the 

 solar rays, may be made so, by previously immersing 

 it for some time in melted borax. See Grossier in 

 Journ. de Physique, vol. xx. p. 270. 

 Optical Since the time of Sir Isaac Newton, the diamond 



properties, has been supposed to exceed every other body in its 

 power of refracting and reflecting light, the index of 

 refraction, according to that philosopher, being about 

 2.439- Dr Brewster, however, has found, that both 

 Chromate of lead and Realgar exceed the diamond in 

 their action upon light. Owing to the great quantity 

 of light which it reflects at both surfaces, the diamond 

 is never completely transparent ; and in consequence 



of its high refractive power, it reflects all the light that Diamoi.il. 

 is incident upon its posterior surface at an angle of inci- **" ""7~ w ' 

 dence exceeding 24° 13', from which cause it derives 

 that high lustre to which it owes its value as an orna- 

 ment. The diamond has always been considered as 

 a crystal which gives single refraction ; and in what- 

 ever way it is cut, it exhibits no direct marks of two 

 images. Dr Brewster, however, has found, that it 

 possesses the property of depolarising light ; and it 

 necessarily follows, from his theory of depolarisation, 

 that, like many other bodies, it actually forms two 

 images, which are polarised in an opposite manner, 

 like those of all doubly refracting crystals ; but in 

 consequence of its possessing only one refractive power, 

 these images can never be separated and rendered vi- 

 sible. The diamond polarises light by reflection at an 

 angle of 68° 10' according to experiment, and at an angle 

 of 68° 2' according to theory ; and its dispersive power 

 is 0.038, nearly the same as oil of olives, and very much 

 below flint glass. 



In India, they are found, in general, in alluvial soil, Gcognostlc 

 and in the same situation in South America. Accord- situation. 

 ing to Mawe, the diamonds of Brazil are found in a 

 loose gravel immediately incumbent on the solid rock, 

 and covered by vegetable mould, and recent alluvialmat- 

 ter. This gravel consists principally of rounded quartzy 

 pebbles of various sizes, mixed with sand and oxide of 

 iron, and containing blue, yellow, and white topazes, 

 and grains of gold. In some parts of Serra do Frio, 

 the gravel is cemented by means of iron into a hard 

 conglomerate. Humboldt observed diamonds imbed- 

 ded in amygdaloid, in specimens, in the collection be- 

 _ longing to the Stadtholder, now in Paris; also in similar 

 specimens in England; and all of them were from India.t 



The diamond was first found in Asia, where it is still Geogra- 

 collected, although not in such quantity as formerly. P mc situ:i - 

 It occurs principally in the provinces of Golconda, t,on ' 

 Visapour, Bengal, and the island of Borneo. 



In America, diamonds occur only in Brazil, in the 

 district of Serra do Frio, where they were first dis- 

 covered towards the beginning of the last century. 



The usual method of searching for diamonds in Bra- 

 zil, is to collect the disintegrated conglomerate, in which 

 they are found, at the bottoms of rivers and of ravines,and 

 by a laborious process of washing as long as the water 

 comes off discoloured, to separate the mud from the dis- 

 tinct grains. The residue thus cleaned, is carefully 

 examined for the diamonds which it may contain. 

 These are distinguished partly by their crystalline form, 

 but principally by their adamantine lustre. Diamonds 

 of the smallest size, that is, whose weight does not ex- 

 ceed a fifth of a carat, or even the fifth of a grain, are 

 by far the most abundant ; these are of no use in jewel- 

 lery, but when broken and ground to the requisite de- 

 gree of fineness, form what is called diamond pmvder ; 

 a material used for cutting and polishing the diamond 

 and other hard gems. 



In some districts in India, diamonds are found loose 

 in the narrow crevices of rocks : the miners make use 

 of long iron rods, with hooks at the ends, and with 

 these they draw out the loose contents of the fissures, 

 and wash them in tubs, in order to discover the dia- 

 monds. In Colour, in Golconda, they dig in a large plain 

 to the depth of ten or fourteen feet, and the earth is 

 carried to the washing places by women and children.:}: 



Method of 

 searching 

 for dia- 

 monds. 



* It was Sir George Mackenzie who first accurately determined the temperature at which diamond begins to burn, 

 son's Journal, vol. v. p. 104. 



■J- Dr Thomson, in his Annals of Philosophy, mentions specimens of amygdaloid containing diamond, from India. 

 J At one period, nearly sixty thousand people were employed in searching for diamonds in Colour. 

 VOL. VII. PART I!. 4 y 



See Nickel- 



