[robbla1981]

the most effective way, as mentioned previously, is to paint a quick sketch, then add in textures from photos of actual rocks. However, I can appreciate the desire to learn how to paint from scratch as exercise. I think the problem is that your brush is way too soft. try using one of the more textured "chalk" brushes (assuming you're in photoshop) with a few different shades to give more of a sharp, rugged texture.