Here is a teaser from the mural project at Zaytoon. How do you design a stone wall to give both the illusion of depth and realism? Be organic with your shapes. Use a watercolor pencil for layouts that way your lines will eventually dissolve into the work and it is also easy to make corrections with compared to charcoal. Think like a mason with larger stones forming your base.
How to: Paint Masonry
If you are painting on a textured surface try using the shapes, peaks and valleys of that wall in your favor for cobbling together realistic dry-fit stone works. Asymmetry is key here enjoy the fact your are not painting a grid of bricks and have fun with your art.