We use natural rocks on our b.p.'s enclosures. I clean them up really well and like Christie said, I make sure they aren't any really sharp points on them. Both snakes seem to use them for crawling on or resting their heads on but since we haven't had either snake long I can't comment on the long term use of natural rock.
Hopefully someone that has kept snakes a long time and uses natural cage decorations (rocks or wood) will comment further.
~~Joanna~~