If you ignore the darks and middle values, you can see that the whole wall of stone has a light gray first layer.
If you make your gray from a warm and a cool color you can adjust the initial wash with touches of those components, creating a soft-edged pattern that begins to suggest facets.
While the initial wash is still wet, add the middle value rust stains and a few soft-edged shadows. Not too many. Better to err on the side of too little information. At this stage of the translation you can shift your attention from the photo to your study, asking whether you have done enough to tell the story rather than if you have duplicated the image.
All that remains are the hard-edged, dark mid-value shadows and the dark, skinny cracks. I have to be especially careful not to overdo these. They are so potent I want to make lots and lots of them, but remember Eliot O'Hara's advice about how many specific little dark marks are enough, "Fewer than half as many as you think".
It is helpful to avoid surrounding individual rocks with dark outlines or hard edges, even if you see them in the photo. Let the wall continue to be a singular thing.
Make a few studies. When you are confident of your sequence of layers, try putting the rock in context.