In several threads on different forums I've come across opposite schools of thought on using rocks (and wood) from "outside." I have known rocks to leech iron, and as this is a mineral within the rock it is impossible to somehow remove it without melting (or more correctly smelting) the rock. Other minerals such as copper may be present and we all know how deadly copper is to fish and plants in excess.
Rocks can absorb liquids and if they come into contact with pesticides, herbicides, oil, and other toxic chemicals it might be months or years before these things leech out completely.
Boiling rocks is not a good idea, they can explode. "Washing" them however thorough is still hit and miss; how do you wash the inside of the rock? I also had interaction with the mining community for many years, and I learned how rocks can contain all sorts of toxic substances.
The opposite school of thought is that most rocks are probably OK, why bother. Years ago I had some rocks from a lake in one of my tanks and I wasn't aware of any problems. More recently I lost a number of fish to something toxic (unknown) that was slowly leeching out of a piece of wood (bought in a reputable aquarium store no less) so now I am a bit more cautious. Thinking that because the rock occurs in a river it must be OK for fish is misleading; in nature the water is constantly moving and carrying away any toxic substances, and fish would avoid the area. In a closed aquarium the fish cannot escape.