Join Date: Nov 2007
Location: NYC & Long Island, NY
The best answer is for you to test an acid on the rocks to see if they'll leach into the water.
They LOOK safe, but that can be misleading. Off the bat, I might be concerned about the reddish material on the rocks. If it's rust, due to metal content in the rock, I'd stay away. Try scrubbing, if it comes off easily, its just dirt.