You should never use anything on a grave stone stronger than water and a very solf brush. If there is lichen on it and short piece of 3/8" dowel cut at a 45 degree angle on one end can be used gently to remove some of the lichen. Mirrors really do a wonderful job if you reflect the light ACROSS the stone from the side to create good shadows. You don't want the light reflected directly from the front. Using chalk, powder, shaving cream or any other foreign substance is a no no. The stones are porous and while you can't see it the powders etc will get in there and further deteriorate the stones. While rain will wash some of the substances away, it will also wash them into the stone. Good luck