Rocks?

1/34 Rocks let rain drain though, if you are constructing a fire ring. Soggy ashes aren't much good. Rain will also dissolve the ashes making lye and draining out assuming you use wood. And air circulating along the ground and up into the fire, make it hotter and burns most or all the wood, a natural draft. Keeping air flowing in under the fire will take some maintenance. At a campground a pair of nitrille gloves is just about a necessity to clean out cans and garbage,dg droppings, plastic everything and glass bottles and cigarette butts. left by the........... Nothing much worse than burning stinking garbage.
As far a large rocks maybe the fire ring is supported on them, a good idea but I haven't seen it.