For the best laser engraving results, polished stone slabs or any stone with a smooth surface is recommend. For an optimal engraving result, the stone should have an optimally flat surface, so the final result is not distorted by curves.
Although, it is possible to laser engrave other stones with 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.