Rocks & Stones from the Beach??

Here is a link I found that might help you
http://answers.yahoo.com/question/index?qid=20070202140926AAJz9PI
I have heard a lot of people boiling drift wood for a long time (30 minutes to an hour)
I personally would not trust it because you don't know what could leach into the water.
This is just clips from a book I have read by Richard E Hess
"Don't use stones such as sandstone, which will disintegrate in the action of the water. Rocks and stones that leach elements into the water, such as limestone and marble, can prove fatal to your fish. Smooth pices of granite and slate are the best choices for aquarium decoration."
Hope this was some help. Good luck on your fish tank.

Try the vinegar test to make sure it's fine.

I read on the skeptical aquarist that the vinegar test is not reliable, and to use your ph solution and if it fizzes its not suitable.
Personally I would boil them, or and soak them and place them into water with ammonia, then soak, then place them into water with de chlorinator (water ager/ prime/ stress coat to seep in.
I soak my things from the beach for a long time, then leave in full sun. Good if you need your water hardened for some cichlids.