790 



DIAMOND 



thus becomes self-luminous in the dark. Under 

 the electric discharge in a highly-rarefied medium 

 it exhibits beautiful phosphorescent phenomena, 

 as pointed out by Mr Crookes. 



The specific gravity of the diamond is about 

 3 '52, and its degree of hardness greater than that 

 of any other mineral, being indicated by 10 in the 

 ordinary scale. The excessive hardness serves to 

 distinguish the diamond from other gem-stones : 

 any stone which readily scratches ruby and sapphire 

 must be a diamond. Notwithstanding its hardness 

 the diamond is brittle, and hence the absurdity of 

 the ancient test which professed to distinguish the 

 diamond by its withstanding a heavy blow struck 

 by a hammer when placed on an anvil. 



Regarded formerly as a peculiar kind of rock- 

 crystal, the chemical composition of the diamond 

 remained for a long time extremely obscure. It 

 was shown by the Florentine academicians Averani 

 and Targiom, about 1695, that the diamond could 

 be volatilised at a high temperature, their experi- 

 ments having been made at the cost of the Grand- 

 duke Cosmo III. 



Newton in 1704 expressed the opinion that the 

 diamond, from its high refrangibility, was probably 

 an unctuous body coagulated. In 1751 the Emperor 

 Francis I. performed an experiment which showed 

 that diamonds strongly heated in a furnace com- 

 pletely disappeared. Towards the latter part of 

 the 18th century several French experimentalists, 

 including Darcet and Macquer, examined the 

 behaviour of diamonds at a high temperature ; 

 but it was reserved for Lavoisier in 1772 to demon- 

 strate that while the diamond is extremely refrac- 

 tory if heated with total exclusion of air, it is 

 readily combustible at a moderate temperature 

 in the presence of air or oxygen, with production 

 of carbonic acid gas. In 1797 the English chemist 

 Smithson Tennant satisfactorily showed the iden- 

 tity of diamond with carbon. In recent years 

 highly refined researches on this subject have been 

 made by Dumas, Stas, Roscoe, and Friedel, all 

 tending to prove that the diamond is practically 

 pure carbon. Chemists have generally experi- 

 mented, for sake of economy, with impure speci- 

 mens, and have thus obtained on combustion a 

 considerable amount of ash, the nature of which 

 has not been well ascertained. It has been shown, 

 however, that the purer the diamond the smaller 

 is the proportion of ash left on its combustion. 

 Gustav Rose, shortly before his death, subjected 

 diamonds, inclosed in a vessel free from air, to the 

 intense heat of the voltaic arc produced by a 

 powerful dynamo constructed by Siemens of Berlin. 

 Under these circumstances the gems became gradu- 

 ally incrusted with a dark coating of graphitic 

 matter, and in some cases triangular depressions 

 were developed on the faces, similar to those often 

 observed in the South African diamond in its 

 natural state. 



As the chemical composition of the diamond is 

 well known, it has often been supposed that carbon 

 might be induced by artificial means to assume the 

 characters of the gem. In 1880 J. B. Hannay 

 of Glasgow announced that he had succeeded in 

 producing artificial diamonds by inclosing a mix- 

 ture of paraffin spirit and bone-oil distillate, with 

 metallic lithium, in a strong wrought-iron tube, 

 and exposing it to prolonged heat in a reverbera- 

 tory furnace. Specimens of diamantoid carbon 

 were thus obtained, but of small size, and it was 

 found that these, when placed on the wheel, imme- 

 diately crumbled. As the experiments offered no 

 promise of commercial success, and were of so 

 difficult and dangerous a character, they have not 

 been repeated. 



It is notable that the occurrence of diamonds 

 has been recorded in a meteorite. Jeofeiff and 



Latchinoff in examining the meteorite which fell 

 on September 22, 1886, near Novi-Urei, govern- 

 ment of Penza, South-east Russia, detected the 

 presence of about 1 per cent, of diamantoid carbon. 



An imperfect variety of diamond known as bort, 

 or boart, occurs in the form of small spherical con- 

 cretions with crystalline texture. Although unfit 

 for use as an ornamental stone, it is employed 

 when ground as an abrading agent ; and the powder 

 mixed with oil serves to feed the lapidary's wheel 

 when cutting hard stones. Much of the bort of 

 commerce is not the mineral ogical variety, but 

 consists of splinters, rough fragments, and imper- 

 fect crystals of ordinary diamonds. The diamonds 

 used by the glazier for cutting glass are ordinary 

 crystals with rounded faces, and are known as 

 vitr6 diamonds. The cut, though only to the 

 depth of about the ^Joth of an inch, is sufficient 

 to determine with precision the direction in which 

 the glass shall break. 



Carbon, or carbonado, is the name applied to an 

 opaque, black, granular, or imperfectly crystallised 

 variety of diamond found only in Brazil, where it 

 was discovered in 1843. While its density is less 

 than that of crystallised diamond, its hardness is 

 much greater, and hence it is used in the arts, 

 especially for mounting in the steel heads or crowns 

 of the rotary diamond drills for rock-boring. 



The art of cutting and polishing the diamond is 

 said to have been discovered in 1456 by Louis de 

 Berguem of Bruges. As now practised, the stone 

 is first, if necessary, cleaved or split, and then 

 ' bruted ' or rubbed into shape. The faces of the 

 stone thus ' cut ' are ground and polished on flat 

 metal discs, fed with diamond dust and oil, and 

 revolving with great rapidity by steam-power. 

 Amsterdam is the chief home of this industry, and 

 the trade is chiefly in the hands of Jews ; but 

 diamond cutting and polishing are also now exten- 

 sively carried on in London, Antwerp, &c. The 

 common form of the diamond is either the brilliant 

 or the rose cut. The brilliant resembles two trun- 

 cated cones, base to base, the edge of the junction 

 being called the girdle, the large plane at the top 



Fig. 1. Square-cut Brilliant. 



is the table, and the small face at the base the 

 culet ; the sides are covered with symmetrical 

 facets. Fig. 1 shows a square-cut brilliant, and 

 fig. 2 a round-cut brilliant. The rose has a flat 



Kg. 2. Round-cut Brilliant. 



base, with sides formed of rows of triangular facets 

 rising as a low pyramid or hemisphere ; but this 

 form of diamond is daily becoming less fashionable, 

 and is therefore of comparatively little value. It 

 is illustrated by fig. 3. 



India was formerly the only country which 

 yielded diamonds in quantity, and thence were 

 obtained all the great historical stones of anti- 

 quity. The chief diamond -producing districts are 



