Rock gardens and xeriscaping ideas are often thought of as in the same design sense. Mention xeriscape design and most folks will picture something like the image below. For those that don't know, xeriscaping is water wise landscaping. The word itself meaning "dry scene". "Xeri" meaning dry and "scape" meaning scene.
While this picture is a good example of a dry scene landscape, it is on the extreme side of what a xeric garden can be. See Xeriscaping Principles and Xeriscape Design Ideas for more information on drought tollerant and water wise landscaping.
The rock garden and xeriscaping example above gets its real beauty and character from the different textures, angles, and colors of landscape rocks. Contrast is created with the lighter colored filler ground cover gravel. Its light color helps define the rocks from one another as well as helps them stand out.