What kind of rock? LOL Lots of different variety of rock out there m8, but generally besides the right colors I just use a stippling action..If I feel more creative a sea sponge works really well, but does have a bit more of a mixed media feel..Start of with the tutorials mentioned, I'm sure there are heaps of vids out there..Also search traditional painting a bit as it basically utilizes similar layering techniques and again no doubt there are lots of videos in that realm as well..GL