CARBONADO  , a name given in

Brazil to a dark massive form of impure diamond, known also as " carbonate " and in trade simply as carbon . It is sometimes called black diamond . Generally it is found in small masses of irregular polyhedral form, black, brown or dark-grey in colour, with a dull resinoid lustre; and breaking with a granular fracture, paler in colour,and in some cases much resembling that of fine-grained steel . Being slightly cellular, its specific gravity is rather less than that of crystallized diamond . It is found almost exclusively in the state of Bahia in Brazil, where it occurs in the cascalho or diamond-bearing gravel . Borneo also yields it in small quantity . Formerly of little or no value, it came into use on the introduction of Leschot's diamond-drills, and is now extremely valuable for mounting in the steel crowns used for diamond-boring . Having no cleavage, the carbon is less liable to fracture on the rotation of the drill than is crystallized diamond . The largest piece of carbonado ever recorded was found in Bahia in 1895, and weighed 3150 carats . Pieces of large size are, however, relatively less valuable than those of moderate dimensions, since they require the expenditure of much labour in reducing them to fragments of a suitable size for mounting in the drill-heads . Ilmenite has sometimes been mistaken in the South African mines for carbonado . (F .

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