The settings rocks on fire thing would be really fun. I wonder if it would work on the rocks around here ...
Depends on the rock composition.
Sulfur bearing rocks will burn, stink, melt, dribble, flake spark etc.
Certain shales and slates bearing organic material will light up, shatter, and burn.
Some pebbly looking conglomerates aren’t, they are concretions containing phosphorus (spelling? Meh, tired) compounds.
So the results of a campfire can be interesting.
Especially if you are known and prone to making HOT fires that can light fiberglass on fire reliably.
(Not that I have done that. )