Adding Ambient Occlusion will also help the realism,but the stone looks too shiny,it doesnt have to be grubby but imperfections add to the effect. also consider the brick textures, look at the other pics given,you can only see the stone/bricks etc when up quite close,a lot closer than your final pic, so that crys out unreal for me,i think you need to get a realistic reference for the size of your stone texture and then perhaps alter the antialiasing of that texture so its a bit more blurry. you want to have hints of bricks at that distance as various parts of them will be lit or shaded,you cant really do that so you have to fake a lil bit