Thats a great idea, saves money and real rocks look better IMHO than fake ones. To prep a rock for tank life, brush off the dirt and dribble some lemon juice on it, to see if it fizzes. If it does, there is metal in it and you can't use it. IF not scrub it down with a scrub brush, or scored pad in a 1:1 bleach/water solution. Be really good if there are any holes in the rock or cracks. Then rise it off with water, really really well. Give it the sniff test. If you still smell bleach rise more.
There are better guides for how to prep an outside rock, but thats the jist of it.