I agree with Shelly. You might be safe with a very pure sandstone or limestone, but limestone may increase your pH depending on what it is right now. I have sandstone in one of my tanks that I got at an lfs. Many rocks will have some concentration of iron because its a very common element. That being said, if I remember correctly, ferric iron (oxidized iron) is not very soluble in water which is why city or well water with high iron will leave orangy-red stains in your tub and sinks sometimes and the toxicity will depend on other factors, such as your pH and Kh. Anywhoo, that is an FYI, and as Shelly said, if in doubt, don't use it
-Sephanie