I was fortunate that in one meeting with a Belgium diamond
dealer, I was able to see a large collection of black diamonds.
They are completely opaque, and cut with a flat back and very large
table facet, much like a faceted "black onyx". While they seem to
be gaining in popularity and quite attractive, I could not convince
myself to purchase any.
Jeffery, Though a flat back certainly makes sense since these things
are opaque, all of the ones I've had to work with, including lots of
melee to be pave set, were normal brilliant cuts. Also it's worth
mentioning that most of what's on the market in these, are the result
of, I believe, irradiation/heat treating, rather than fully natural.
The fully natural ones are just cut from diamonds which are so full
of the typical "carbon" inclusions or other dark inclusions as to
appear opaque and black. I'm not sure what the treated ones start
out looking like...