“In this tutorial, we will exercise on drawing rocks, stones, cliffs or anything made of stone. There are many ways to achieve this… I will just show you my favorite one. depending on their structure, rocks look very different. For this tutorial I decided to make sharp reefs in the water … but you can use the same technique for anything stoney.”
http://www.3dtotal.com/team/Tutorials/painting_rocks/rocks_01.php