The type you're thinking of is tension set, which really does make a neat ring. For anyone who hasn't seen one, you basically take a very strong ring, slice it, pry it open, and insert the gemstone in, and let the ring snap back to hold the stone. It only works with hard stones that it won't crush. It's supposed to be good for active people, as the stone is held by a lot more force than the little bended prongs in a traditional mount.

Matthew