for a rocky surface
i would use some polyurethane foam and then a sharp knife to form the foam
i have used some spackle to fill the rought surface of the cutted foam in one of my mods, and it works well
now i would use different colors in a paint technique
-> accentuate
- ground with a dark paint
- slightly paint highlights with a bright color and few paint on the pencil
- finish with some clear varnish
have fun ....