That depends on what you want to do! For cobblestones I\'ve had some success by using green stuff flattened out to a thin sheet which I covered the base with. With a modelling knife I sculpted the shape of the cobblestones and made a few cracks in the stones. I then took a flat stone and pressed it onto the green stuff to make a textured surface. Some tidying up with the modelling knife and then it\'s done!
Otherwise, cork is good for rocks, real stone is useful, tree bark etc. You can use a harer putty, milliput for instance, and make a flat disk out if it, then smash the disk into small pieces (once it\'s fully cured) and use them as jagged rocks.