No stones. They wind up coming up to the surface. I know two instances where this was done and within a year, what a mess. Lots of stone picking went on. You don't have to do the whole pen, just a 4' path about a foot in from the panels. As the horse moves it will go outward anyway. That can be over come with plywood or boards along the bottom. Course sand is nice. Keep the horse off it in wet weather so as not to churn the sand into the clay. If you're careful it should be good for about 5 years before needing new sand. You might have to use Roundup to control grass if it comes up too thick. A 40' ring is a bit small for a horse, 50' is better.