Hey all. I’m building a fire pit for my back yard and I’m looking for recommendations on what kind of rock to use.

My pit is going to consist of 2 concentric metal rings, one at 48" wide and one at 36" wide, leaving a 12" gap between them. I’d like to fill this gap with some kind of rock (and maybe cap it with flagstone) but there’s not any good information about what kind of rock is ok for that purpose.

I understand that for the base, I’d want to use either sand or lava rock, and if i was just surrounding the inner ring I’d need heat bricks or something similar, but I’m stuck on what can go between the rings without worrying about explosions.