I wasn’t happy with the basalt rock so I watched a few tutorials by Shaddy Safadi (this guy is awesome) and then went back to the blocking in stage and started again from there.
- Initial block-in:
2. Blocking in the accents: 
3. Using Lock Alpha Channel with a textured brush (Acrylic 2) to give the accents some texture: 
4. Blocking in the highlights: 
5. Locking Alpha Channel and using a textured brush on the highlights: 
6. I tried to add a bit more texture to the accents layer with a large textured brush and a selection around the rock island only: