1911 Encyclopædia Britannica/Carbonado

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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. W. R.*)