The age of these diamonds is between 2.6 billion and 3.8 billion years, dating to a time when South America and Africa were joined. No carbonados have been found anywhere else on Earth, leading Haggerty to believe they all originated from a single asteroid impact. But he doesn’t think that one giant carbonado fell from the sky. All the stones are opaque and porous, but their colors vary from black and gray to green and even red, which tells Haggerty that it is more likely they arrived here embedded within another rock. The matrix that held them has now weathered away, leaving a scattering of stones.The largest one reported is the Spirit of de Grisogono diamond, pictured above in a photo from Sybarites, (set in a ring worthy of a professional boxer).
...the world's largest cut black diamond and the world's fifth largest diamond overall. Starting at an uncut weight of 587 carats (117 g), it was taken from its origin in west central Africa and cut by Swiss jeweler De Grisogono. The resulting mogul-cut diamond weighs 312.24 carats (62.45 g) and is set in a white gold ring with 702 smaller white diamonds totaling 36.69 carats...Some sources I've read say that black diamonds are porous and not polishable. In fact, per Science Daily, "The term carbonado was coined by the Portuguese in Brazil in the mid-18th century; it's derived from its visual similarity to porous charcoal."
The photo above seems to contradict that description. I'm not sure why the discrepancy exists.
Addendum: I had forgotten that two years ago I wrote a post about an 11th century ring containing a black diamond.