I've mucked around doing these but my skill is sub-par. I do them in wood and Tagua nuts (vegetable ivory). It might be hard to find ivory to get carved, bone is easy enough (large bones from the pet shop), antlers aren't too hard to find, shells are everywhere but I think the problem there would lie in finding someone with the knowledge to do the work in that medium.
If you get it sculpted, it would probably be a mix of white and translucent polymer clay that gives the right look, maybe with a touch of sand/tan/etc if you want an aged look. Although, I guess you could get it sculpted in any medium if you were going to get it cast in porcelain or alabaster.
Cast in metal, as a keychain, might be a cool idea though.