If rocks are too heavy, you can make your own from things you have at home. For a school project I once wadded up tissues and paper towel in glue, smushed them into rock shapes to stick to the wall of the little diorama of a proposed museum exhibit, and painted them when dry. It's a lot lighter, and you can make any shapes you want. Add a bit of mica powder if you want some shimmery rocks like the kind we find around here for making campfire rings.
...and of course you need to burn some of the sticks to make it look like the fire's gotten to them!
Thanks for the great suggestions icekibby. I was
thinking the fire ring might get a little heavy.