Safe aquarium rocks? And how to test?

Hi,
I was wondering if the rocks that I have been getting from the bank and bottom of my creek is safe for an aquarium. I am planning on using them for cave making in my 55G. Some are thin and flat, round, thick and square. Is there any way to accurately test them?
Thanks in advance!
Answers:
There are a few tests you can do. First you should get them cleaned up so it's the rocks that you'll be testing, not anything growing on them. Put them in some hot water and scrub them a little with an old toothbrush or something similar, then let them air dry.
Once they're dry, put a few drops of vineger on them and see if they fizz and produce bubbles. If so, these have carbonates in them and they'll raise your pH. These wouldn't be good to use unless you have cichlids that need higher pH water.
Rinse the vinegar off, then put them into a clean container of water for about a week. After the time is up, look to see if there's an oily film on the water. If you can, it's also a good idea to test the pH of the water they were in (a pet store can also do this) and compare this to the pH of the water source. If the pH of the water the rocks were in in below 6 (and your water shource isn't), you don't want these in your tank - some sandstones contain small amounts of pyrite (fool's gold), and this will make your water too acidic.
Other than the rocks you've eliminated with the above tests, they should be okay to use.

---

http://saltthesandbox.org/rocks/acidtest...
You generally want to stay away from rocks that contain calcium carbonate. This site will explain how to test them.