Most of them are inert but unless you know the exact make-up of the sandstone (what kind of sand and other minerals are in the rock) then I wouldn't use it. Some sandstone will just disintegrate in water while others are fine. Test them first in buckets of water, one per each. Check the rock for any signs of crystal forms or anything metallic--those would be toxic. Check the pH 24 hours after soaking and check the rock for any sort of disintegration, also if you can check calcium levels, iron, and--well basically just check everything you can, Hardness and Alkalinity too to see if it changes the water.