It depends on the size of your fire. A small cooking fire works best ringed becase you can use the rocks to rest coffee pot, etc. on and keep them warm. A ringed fire also seems to build up a really hot bed of coals faster than an unringed one.

For a large fire to generate heat through the night, a ring is impractical, especially if you are burning long branches or logs and keep pushing them into the fire as their ends burn off.

Chris is right about sedimentary rocks. Choose igneous and watch out for deep cracks. Having said that, I have never seen a rock explode, but I've seen lots crack in half.