Wow, where to begin...

There use to be a few decent rock generator scripts. Many have gone the way of the Dodo.

Here's one: http://www.turbosquid.com/FullPreview/Index.cfm/ID/272737

Another: http://www.programmersheaven.com/download/41674/download.aspx

Another: (scroll down) http://www.inf.tu-dresden.de/index.php?node_id=992

Or you could use Poser's displacement feature on a sphere (with a noise map)