Here are a couple of images of a tiling rock wall that I did for Sega Rally Revo. They were very specific on what they wanted here, provided photo source, etc. Before anything else, have some reference in front of you and get a good idea of exactly what kind of rock that you are going for. There's a wide variety of rock types and surfaces. What you'll see in a river in eastern Europe is a lot different from what you'll see in a desert in Arizona.
Technically, make sure that you've got enough geometry to carry the profile well. After that normal maps and AO bakes can really sell things.
In this case, the photosource had quite a bit of lighting already in it, and it wasn't worth doing an AO pass on it. It was actually a fairly easy cut by numbers job, where I just traced cut lines around the recesses, then added extra verts in the middle and pulled them out. After that, it was a CrazyBump pass if I remember correctly.