In 2010, R. Crumb and company released an edition of 40 hand-carved sandstone statues of Mr. Natural. They had experimented with several carvers and different types of stone, and while they chose the sandstone...there was a very small run of prototype "volcanic" Mr. Natural statues created using volcanic rock.
The volcanic Mr. Naturals are not part of the limited edition statues. The statues are sealed and hand-painted. They are marked as prototypes and limited to only four pieces running $350 plus $60 shipping (US only).