Grandma's rock in aquarium?

Umm...I am not sure if it would be safe or not. It seems like we put a big rock in our 55 gallon aquarium, but i don't know what kind it was. I don't think our fish got damaged... and for the rock, prob. the only damage would be fish poop. If you do put it in there, make sure you clean it...just dont use soap. But if you want to be on the safe side, you might not want to do it. But up to you. :)