Sentence:  At present, equal attention is paid to irregular fragments of a blackish or greyish colour, occasionally of considerable size, also yielded by the washings of diamandiferous sand, which formerly passed unregarded. These fragments are now carefully colected, and have acquired some considerable value in commerce, where they are known under the name of <WSD>"carbonado</WSD>" or carbon. [...] An examination of these numerous varieties has made it evident that between <WSD>"carbonado</WSD>" of a simply micro-crystalline texture, and the diamond regularly crystallised in diaphanous octahedrons, there exists an uninterrupted series of intermediate conditions. 
Meaning of the word marked with <WSD> tags:  A dark, non-transparent, impure form of polycrystalline diamond (also containing graphite and amorphous carbon) used in drilling.