Hmmm. Well, I set up an afrcian tank recently using pool filter sand, a large plastic plant, a small plastic ship, and some rocks. The rocks are nice. I found them while hiking along a trail alongside one of the locks for Lockport Canal. I found a rock, completely black with some brown and red streaks, thats double the size of my fists combined but almost as light as a few marbles!
I tested them with vinegar and lemon juice, the rock test stuff I heard about, and cleaned them. So far, no problems. The fish love them. It worked for me.
Try it; the worst that can happen is it may leach something or affect the ahrdenss or ph. If it does, remove it. But if not, well, it beats anything from any store anytime cause its free!