These are cast in pigmented stone, possibly hydrocal, maybe ultracal. They feel really great.
The weight on them is very nice, and the detail is great. I beleive they are cast from screen used stones, as I had a chance to recently see the genuine article.
My only complaint about these stones is that they bow a little tiny bit. It's very subtle, and you would probably need to be looking for it, but there is some gentle bowing to each stone. Oh well.
All in all, they are really great. Now, if I can just find a Leeloo to go with them.