Have already used CLR with very little improvement. TY
Hi Donna, here are a few ideas for you to try, hope they help you
Donna....Try Krud Kutter from Lowe's
Could be its lichen growing on your stone,,,, Wet n Forget is one commercial product for removal. Here is a discussion on the topic..... https://inspectapedia.com/exterior/Stone_Cleaning.php
Have you tried power washing it?
Start with a solution of warm water with vinegar and dish detergent. Apply to rock and scrub any areas that are very dark. Rinse well with garden hose. You might want to get a high pressure wand or a pressure washer to rinse.
Take a good photo to the home improvement store and ask for their advice...they have products you may have never thought of.