I clean gravestones and memorial plaques using products and practices that remove dirt and moss without harming any type of stone or metal. Chipped, gouged, and scratched surfaces can also be repaired. A couple hours is all it takes. Products are non-toxic and biodegradeable.
The process starts by moistening the surface with water. This particular gravestone is granite. Then I spray a biocide and let it soak for a few minutes. Using a special accessory attached to my cordless drill I begin to scrub off the dirt and moss. This tool alows me to clean all the nooks and crannies without scratching the surface. Finally, I rinse it all off with water. If needed I repeat the process, but in general, one pass is all it needs.
This is a great example of a heavily weathered stone that cleans up very well. If the stone is marble I add a protective coating to help the stone past longer - it's not as hard as granite and thus does not last as long outdoors without a protective coating.