If sandstone is what I think it is, I would say its far to porous to use as a substrate. Slate works great and other hard non porous rocks. Just wash completely and disinfect. The less pits and deep cracks the better though, so with slate, make sure there are not any loose 'flakes' of stone, where germs and bacteria from waste material can get into. Soaking/disinfecting the stone every few months is also a great way to prevent build up of odors or bacteria in cracks in the stone. Fortunately, slate and other non porous rocks are easy to disinfect and clean.
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PHLdyPayne