I would suspect that true rocks are fine. Granite especially should be absolutely fine. I would be more concerned about sharp edges, and mineral rocks. The other thing is when you find a rock you want, bring it in and boil it to sterilize any bacteria, fungus, or parasites that might be on it.
I don't have many fish, but my dad has several goldfish, he found a rock and plopped it in the tank a few days ago.
I have fire bellied toads, (hence the name FBT) and half of my tank is constructed with rocks I found. Doesn't bother them.
Just look at all that rock!
http://www.youtube.com/watch?v=J6vRK6qJIJ4
I don't have many fish, but my dad has several goldfish, he found a rock and plopped it in the tank a few days ago.
I have fire bellied toads, (hence the name FBT) and half of my tank is constructed with rocks I found. Doesn't bother them.
Just look at all that rock!
http://www.youtube.com/watch?v=J6vRK6qJIJ4