Boiling rocks isn't a good idea.
I put rocks in a bowl and pour hot water over top of them and let them soak for awhile then I scrub them really well and lay them out to dry in the sun for a day or two.
Make sure the white rock isn't limestone, It could mess with your PH.
You can use coconuts, legos, mugs, terra cotta pots and I have even seen a few people use PVC pipe as caves..I use glass jars from olives and such and I cover it in aquarium glue and stick the gravel around it so it blends in with the gravel. :)