i am building a big new enclosure for my savannah monitor, and plan on attaching some big rocks together to make a good sized cave for him. rather than spending money on rocks from a store somewhere, i'd like to collect them from the outdoors, as i know several good spots to grab some...is there anything i need to do to sterilize them and make them safe for my lizard? hope it's not a dumb question, i would just hate to screw up and have something bad happen.. any help would be appreciated