CG3DANK has a thorough tutorial on sculpting rocks in ZBrush for you to check out.
In this tutorial, I am telling you the full procedure of rock sculpting step by step,specific brushes used for sculpting and how to create preset of rocks in form of alpha.
The channel is full of other useful tutorials, so make sure to check it out for more videos on ZBrush, Blender, Mudbox, 3ds max and other popular tools.