166 REPORT OF NATIONAL MUSEUM, 1899, 



forms the name hlack diamond^ hort^ and carbonado are applied. (Speci- 

 mens Nos. 53668-53671, U.S.N.M.) 



Origin and Occurrence. — The origin of the diamond has long been a 

 matter of discussion. A small proportion of the diamonds of the 

 world are found in alluvial deposits of gravel or sand. In the South 

 African fields they occur in a so-called blue gravel, formed, according 

 to Lewis, along the line of contact between an eruptive rock (perido 

 tite) and highly carbonaceous shales. They were regarded by Lewis 

 as originating through the crystallization of the carbon of the shales 

 b}'^ the heat of the molten rock. De Launay states, however, that 

 there is no necessary connection between the shales and the diamond, 

 and shows with apparent conclusiveness that the latter occur often in 

 a broken and fragmental condition, such as to indicate beyond doubt 

 that they originated at greater depths and were brought upward as 

 phenocrysts in the molten magma at the time of its intrusion. The 

 primary origin of the diamonds he regards as through the crystalliza- 

 tion, under great pressure, of the carbon contained in the basic magma 

 in the form of metallic carbides. 



The diamond-bearing rock as above noted is a peridotite often brec- 

 ciated and more or less serpentinized (Specimen No. 62108, U.S.N.M.). 

 The blue and green gravel formed b}^ the decomposition of this rock is 

 shown in Specimen No. 73188, U.S.N.M. With these are others of the 

 associated, eruptive, and metamorphie rocks, as melaphyr (Specimen 

 No. 73184, U.S.N.M.), quartzite (Specimen No. 73185^ U.S.N.M.), 

 shale (Specimen No. 73186, U.S.N.M.), and basalt (Specimen No. 73187, 

 U.S.N.M). 



Whether or not a similar origin to that outlined above can be attrib- 

 uted to the Brazilian diamonds is as yet unproven. Their occurrence 

 and association with detrital materials resulting from the breaking 

 down of older rocks, with which they may or mav not have been 

 originally associated, renders the problem obscure and difficult of 

 solution. 



According to Kunz,^ 95 per cent of all diamonds at present obtained 

 come from the Kimberly Mines, Griqua Land, west South Africa; of 

 these, some 47 per cent are bort. The remainder come from Brazil, 

 India, and Borneo, A few have been found in North America, the 

 Ural Mountains, and New South Wales, but these countries are not 

 recognized as regular and constant sources of supply. 



Uses. — The material, aside from its use as a gem, owes its chief value 

 to its great hardness, and is used as an abrading and cutting medium 

 in cutting diamonds and other gems, glass, and hard materials in gen- 

 eral, such as can not be worked by softer and cheaper substances. 



With the introduction of machinery into mining and quarrjang there 



^ Gems and Precious Stones. New York, 1890. 



