Copying other painters rocks might light the way, especially if the artists have found effective ways to simplify the subject. George post comes to mind, and Sargent. Hopper, Wyeth. Painters who do a great job of elaborating rocks, like Stanislaw Zoladz or William Trost Richards, might also open doors for you, even just to borrow their shapes.
Right now the sun is out, and there's the promise of sun in the forecast. How about going out to look for one well lit rock that you can paint several times till you can make one without looking at the actual rock?
Here are a few images to get you started. Looking online for more images would be time well spent. At the least you'd get a better idea of what you like. Have fun!