Then choose the Clay tubes brush and start adding forms to the mesh (Fig.04). You can play here as much as you want. The problem is that the results always look cool and you can spend days messing around. That's the problem of ZBrush; once you’ve started, you can't leave it alone!
So, decide what exactly you want and the result should look like Fig.05.
It’s now time to add some details on the base pattern of the rock. In this case I am choosing the Crumple brush. Use this brush sensibly because if you sculpt a couple of times in one place then you can get bad results with the polygons below. Then this result will appear in your normal map. So to avoid these poly stretches, use the brush easily (Fig.06 – b).
Ok now you can leave your model as it is; it looks good and I’ve achieved very nice looking results in a game engine by only using these two brushes. But in this case I am going to use the noise feature in ZBrush. Go to the Tool menu and open the Surface tab. Check the noise button, increase the noise scale and decrease the noise strength. Then you can play with the noise curve and you will see the changes on the mesh. When you like the result click the Apply to Mesh button. Too much noise can destroy the form of the object so be careful (Fig.07 – a).