this rock boiling controversy we get into (every single time this question is asked, by the way) reminds me of a recipe i came across once. it was called Danger Pudding. you simply get a can of sweetened condensed milk, and throw the whole unopened can into a pot of boiling water for like 20 minutes or an hour or something. it might explode (Danger!) but if it doesn't, you end up with a nice can of pudding.
being the daredevil that i am, i just put some vinegar into a water bottle and heads out into the wilderness. if a rock fizzes, i leave it. if it looks rusty, i leave it. if it falls apart when i pick it up, i leave it. when i get home, i rinse them off in the sink and into the tanks they go.