CAEBON AND THE OXIDES OF CARBON 329 



compounds found in plants and animals have already been men- 

 tioned. The mineral oils consist almost entirely of mixtures of 

 various compounds of carbon and hydrogen (hydrocarbons). 

 Whole geological formations are composed of carbonates of common 

 metals, particularly calcium carbonate or limestone. 



Diamond. This aKotropic modification of carbon is dis- 

 tinguished by its natural crystalline form, which often resembles 

 the octahedron (Fig. 39, p. 94) . The ultimate struc- 

 ture of the diamond crystal is represented in Fig. 45 

 (p. 96). Its specific gravity is 3.5. For ornamental 

 purposes the diamond is " cut " by grinding new faces 

 so as to give artificial forms called " brilliants " (Fig. 

 86) and " rosettes." It is the hardest of familiar 

 substances, and can be scratched or polished only 

 by rubbing with diamond powder. The colorless 

 stones and those with special tints are valuable. 

 The black (" carbonado ") and badly colored specimens are less 

 valuable and are used for grinding, for glass-cutting, and on the 

 points of drills. 



Diamonds are found chiefly in South Africa and Brazil. They 

 are separated from the rock by weathering and washing. They 

 are sold by the carat (1 international carat = 200 mg.) and the 

 value increases with the size. The largest known specimen, 

 the Cullinan, weighed 3032 carats before being cut. 



Small synthetic diamonds are obtained when molten iron con- 

 taining dissolved carbon is suddenly chilled. If the fused mass 

 is allowed to cool gradually, however, the carbon separates out 

 in the form of graphite. 



Graphite is found in nature in Siberia, Cumberland, Brazil, 

 Ceylon and elsewhere. It forms dark grey or black hexagonal 

 tablets, and, when pulverized, it gives slippery scales of micro- 

 scopic size. Unlike the diamond, it is quite soft, has a specific 



