Polished stone slabs or stones that have a very smooth surface will provide the best laser engraving results. For an optimal engraving result, the stone should have an optimally flat surface, so the final result is not distorted by curves.
Of course, you can also laser engrave “real” stones with a rounded surface. To this end, place the stone on a horizontal plane. Use a support for this, such as modeling clay. When focusing, use an average value of the area to be engraved, so the engraving becomes as uniform as possible.