174 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



weighing from 700 to 800 carats have been occasionally met with. They sometimes have 

 the appearance of being fragments broken from a larger mass, and some show a fine striation 

 similar to that of fibrous coal ; this latter feature is believed to be due to friction between 

 several fragments. 



The surface lustre of carbonado is dull and sometimes slightly greasy ; the interior of 

 the nodule is usually rather brighter and shows numerous brightly shining points. The 

 colour of the exterior always lies between dark grey and black ; a fractured surface is a little 

 lighter in colour and shows a tinge of brown, violet, or red. 



This substance is but rarely absolutely compact, almost invariably it is more or less 

 markedly porous, so that it is very similar in appearance to coke. When heated in 

 water, numerous air bubbles are expelled from the spaces in the porous material. Its 

 cohesion is usually considerable, but some samples are easily powdered. A microscopic 

 examination of the powdered material shows it to consist of very small octahedra of 

 ordinary diamond, usually semi-transparent, and containing many small opaque inclusions ; 

 they are nearly always of a light brown colour and only very rarely water-clear. Carbonado 

 is therefore nothing more than a finely granular, porous to compact aggregate of minute 

 crystals of diamond, and is not, as is sometimes incorrectly stated, amorphous diamond. 

 It differs also from the black diamonds which occur at some localities in regular crystals built 

 up of a uniformly compact substance. Some specimens of carbonado aggregates are 

 penetrated in places by ordinary diamond of a lighter colour, and having the usual 

 strong lustre and non-porous character. Cases are also known of the enclosure in a nodule 

 of carbonado of a small, simple, colourless crystal of diamond, the compact substance of . 

 which passes gradually into the porous substance of the carbonado shell, just as do the 

 streaks of paler coloured diamond which sometimes penetrate the dark carbonado. The 

 walls of the cavities in the porous carbonado are sometimes, though rarely, encrusted 

 with minute colourless crystals of diamond. 



The largest specimen of carbonado known was found July 15, 1895, in Bahia, in the 

 neighbourhood of the town of Len9oes, between the Rio Rancardor and the stream known 

 as the " Bicas." It is about the size of a man's fist, and when first found weighed 

 3167 carats ; since it was taken from the ground it has gradually lost 19 carats in weight, 

 so that its present weight is 3148 carats, or about 650 grains (nearly 1| pounds avoirdupois). 

 The heaviest specimen previously known weighed only 1700 carats, and was of inferior 

 quality. 



The essential constituent of carbonado, as of ordinary diamond, is carbon ; the former, 

 however, contains a much larger amount of impurity than does the latter. After combustion, 

 this impurity remains behind as an incombustible residue, and sometimes forms a skeleton 

 outline of the original fragment or nodule of carbonado. The amount of incombustible ash 

 varies from ^ to over 4 per cent, of the weight of the carbonado burnt. Three specimens 

 examined by Rivot contained 96'84, 99-10, and 99-73 per cent, of carbon, and 2-03, 0-27, 

 and 0-24 per cent, of ash respectively. This ash resembled in appearance a yellow, 

 ferruginous clay, and enclosed microscopic crystals of an undetermined substance. By treating 

 finely-powdered carbonado with aqua regia a portion of the mineral matter constituting the 

 ash may be dissolved out, the solution being found to contain iron and a little calcium, but 

 no aluminium or sulphuric acid. Dana gives as the composition of carbonado : carbon 97, 

 hydrogen O'S, oxygen I'S per cent. ; the presence of the last two constituents, however, 

 requires confirmation. The view has been expressed that carbonado is a mixture of 

 crystallised and amorphous carbon, but it is not supported by a microscopical examination 

 of the material. 



