These are lab created black moissanite facet rough. We have a few similar blocky pieces available by weight range as well as individual pieces labelled with a letter and weight which can be purchased individually. These are a deep, ultra dark black, showing very very slightly green if brightly illuminated on a very thin corner.
This is moissanite, aka silicon carbide, facet rough. Though it is often thought of as a diamond stand-in this material was actually grown for use in computer chips--fortunately it didn't quite meet spec and we were able to get our hands on it instead. Silicon carbide is much like diamond with great durability and even greater luster, meaning that a black moissanite is even brighter than a black diamond. If you're looking for the ultimate in black gems this is it!