Choosing rock from a body of water that is exposed to wild amphibians, fish or reptiles is a recipe for disaster. The risk of transmitting parasites, fungi, or disease is very, very high. The risk of contamination due to run off pesticides is also very high.
The ONLY safe way to choose rock is to identify it first. Many rocks in themselves can be toxic or cause problems with water chemistry, thus becoming detrimental to you pets.
The only safe procedure for treating rock, be it found or purchased is a heavy scrubbing with a solution of salt and hot water, followed by extensive boiling.
I do not want to discourage you (I use "wild" rock myself), but I must state you should prepare and be prepared in order to protect your pets.
A bit of work here with the site search function will give you even more details!
There is also an article that while it is on substrates, will give insight on many of the rules for choosing rocks too.
Caudata Culture Articles
(Shame on me for still not finishing the rock articles...)
