You can. We just had a wildfire here and in a few spots, where the rocks formed a natural chimney filled with flammable material, the rocks have vitrified and d

You can. We just had a wildfire here and in a few spots, where the rocks formed a natural chimney filled with flammable material, the rocks have vitrified and deformed. You just need intense heat for a prolonged period - if they could do it with mounds of gorse, it can still be done.