Paint the entire wall with a natural base color, such as brown or gray. This will be the base color of the stones and will also be the color of the "mortar" in between each painted stone.
Allow the base coat to dry, then tape off the wall with painter's tape to form rock shapes in whatever pattern you want. They can be round, square or rectangular.
Dab the slightly darker paint color onto the wall using a sea sponge. Concentrate on each stone individually to make sure they're each covered with the top coat of paint.
Dab the still-wet paint with a cheesecloth to give it a textured stone look.
Allow the paint to dry and remove the painter's tape.
Dip a small paintbrush in the darker paint shade, and paint around the edges of each of the individual stones to give the illusion of depth.