I like large stones to look more rustic than brick, but it sounds like you want something very close the the ground? Will peeler need to step over it to get to the fire pit?

What about wood or steel like https://www.gardeners.com/buy/corten-steel-landscape-edging-5-inch-set-of-3/8599845.html.

Bury some old/used bricks on end, low enough to mow over, high enough to keep the rocks in.

We have a boulder firepit in a gravel ring. I'd like to put some kind of edging around the pea gravel to keep it in the ring and not out in the grass area. We have a pretty natural/rustic looking yard and a lot of the edging I see wouldn't really fit with the naturalistic look I'm trying to achieve. Can you recommend some other type of edging that is natural looking (ie, not a paved patio type edging), and not too expensive?