Burnt mounds an stuff.
I'm probably talking crud*, but I reckon the stones in burnt mounds would possibly be rounded 'cos they would be likely to be dense igneous stuff that only weathers when rolled in rivers etc. The reason they'd end up in burnt mounds is that they'd have thermal retaining type properties. I suspect there would be some discoloration on the surface due to repeated heating and cooling.

* This is 100% conjecture, based on dodgy observation, rather than any kind of reliable evidence or suchlike.