Why bother designing something that has been designed by somebody else, go to 3D Warehouse: http://sketchup.google.com/3dwarehouse/search?q=boulder&styp=m&scoring=t&btnG=Search , type in boulder and press enter. A range of rock designs are available. A great thing about Rhino is that it will open any Sketchup 6 drawing, so look for a rock with "open with Sketchup 6" and download direct to Rhino. You will end up with a "Mesh" rock.
Explore Sketchup, if you see something you want to download that is Sketchup 7 or 8 then just download Google Sketchup, open the file in Sketchup and then save it as Sketchup 6 and then open with Rhino.