Last week in the Tuesday watercolor class that I teach at the Oak Ridge Art Center we focused on painting rocks. Painting with watercolor does not provide real texture so we used fun ways to create the illusion of roughness on the rocks. A piece of a credit card can be used to scrape in to the wet paint for the overall rock shapes and you can dry brush and spatter paint to add more texture. Collage is also a fun way to create real texture in the rock areas of your watercolor.
You can see more of my paintings with rocks in them here