This looks like charcoal, but is actually an uncut black diamond from Sierra Leone. Gem-quality black diamonds are rare; they are mainly used in industry because of their ability to cut the hardest materials.
This is a "rare brazilian ballas" according to the diamond trade - this is a fascinating spherical diamond with no crystal form and a smooth surface like a pearl. It is entirely translucent. Ballas are almost impossible to cut because even other diamonds have serious trouble cutting them. (3.75 carats)