I was recently hired to clean some gravestones in a cemetery outside of Cashton, WI. It was in that cemetery that I encountered the most stubborn biological growth I have ever tried to clean. In my 12 years of gravestone cleaning, I have never encountered algae coverage as black, thick, and resilient as found on this marble gravestone, especially on the backside. My first cleaning turned the black into gray. The D/2 Biological Solution I use to clean and treat gravestones continues to work for several weeks as it kills the stuff growing on the stone. While I know that the algae has permanently stained the marble, I'm hoping that once the algae is killed off, it will lighten up even more. I'll provide an update the next time I'm able to stop by that cemetery.
I follow professional cleaning guidelines of "DO NO HARM" to self nor the gravestone while cleaning. I suppose if I blasted the gravestone with a power washer and grinded it down with a metal bristle brush I would have better results. But I'm in this business to "save stones," not just make them look nicer for my clients.