Very inspiring topic !
I mainly used crease ( add mode as much as sub ), pinch and polish brushes.
A little bit of clay strip here and there, and a final pass with the draw brush with a noise texture to add some grain.
I tried to use the masking tool to bulge out some specifically shaped cracks, but the results were not very convincing.
Bottom line, sculpting rocks is a lot of fun ! But it's quite hard to call it done ...
PS: I also very much like the stoney bear head.
