Make a big brush with a rocky texture on it.
Use the clip tool and slice off all the corners at odd angles.
ROCK!
Here's a picture of me making the last slice on my rock:
Hint: Slice the corners in ALL THREE 2d views.
HINT 2: Never use the clip tool for non-natural stuff. It's ok for rocks and debris, NOT for everywhere. It can corrupt your map BAD if overused.