If I had to make something, I'd probably go with strofoam, carve it to shape, than coat it in bondo or other automotive body filler.
If you need extra durability, you could go with an epoxy & fiberglass layer over the syrofoam, before you applied the auto filler:
- first, make sure you don't mind ruining whatever you're wearing
- cover your work surface, as epoxy cleanup sucks.
- cut strips of fiberglass
- Mix a batch of slow cure epoxy (not the 5 minute kind)
- paint the epoxy on the surface
- press the fiberglass into the epoxy (wear disposable gloves)
- paint more epoxy over the top of the fiberglass
- repeat with the fiberglass going a different direction.
- end with a layer of epoxy
- let cure overnight, or however long the epoxy instructions say.
- cut the item free from whatever surface you left it on.
I've never painted something with a rock finish, though. I've seen it done on home decorating shows, generally using sponges or crumpled paper (I admit it ... I used to watch Changing Rooms when it was on ... I wasn't so much a fan of what the Americans did with it ... well, Vern's designs were usually good, the rest all basically had a set style, and kept doing the same thing over and over again)