In this recipe, we will create a realistic rock material similar to the one shown in the following screenshot:
The best way to start this task is by using image maps. Image maps are particularly useful for several reasons: they already have the necessary color information of a natural surface ready to use. They can be easily edited in any image editor to obtain different information as, for example, high levels for the bump maps. They are processed faster than procedurals by the software (procedural textures must be calculated every time). Moreover, they can be easily found for free on the Web in several sizes and resolutions.
Just remember to make any image you are going to use "tileable" using your preferred image editor. In Gimp, this job is automatically done by a plugin under Filter | Map | Make Tileable.