hhhmmmmm... it kinda looks like limestone to me. if I were you, I would put one piece into a bucket of water, and then get some white vinegar and pour like a teaspoon on another piece and see if there's a reaction. if there's no reaction, I'd rinse it off really well and check it with ammonia... if still no reaction, you'd probably be safe.. but I would still check the PH of the water after a few days with the rock in it. - probably want to pour a glass of water with nothing in it at the same time and do a PH test on the water in both the glass and the rock, just to have a... whaddotheycallit... a Control. Yeah, to have a Control.