The Collaborative International Dictionary of English v.0.48:

Carbonado \Car`bo*na"do\, n.; pl. Carbonadoes. [Pg.,
   carbonated.] (Min.)
   A black variety of diamond, found in Brazil, and used for
   diamond drills. It occurs in irregular or rounded fragments,
   rarely distinctly crystallized, with a texture varying from
   compact to porous.
   [1913 Webster]