I've just started reading a very interesting article on rocks in the new issue of Aquarium Fish Magazine (AFM). There are two tests to use when determining whether or not to use a rock in an aquarium. First use a knife to try and gouge the surface of the rock (prefferably the rock should have no buildup on it). If you are able to gouge the rock it is no good. Now, if the rock passes that test, use the acid test. Use a knife and scrape a portion of the rock's surface. Then drop some acid on the scraped surface. While it is possible to use vinegar, it is often not strong enough to create a visible fizzle on bad rocks. Therefore, a stronger acid is suggested to be more accurate. If you see fizzling, the rock is no good. If it passes both those tests then bake or boil and clean the rock and its good to go. I plan on using this method tomorrow when I collect rock at the stream by my house.