All around us you can find stuff that works great in our modeling world. Our buddy Christian kept telling us to make sure to tell everyone about the great virtue of woodchips that you can find in any park or garden.
Look for some good pieces, paint them black and with a bit of drybrushing, you have a beautiful rock. Have a look at this Ork, standing proudly on top of his wood rock.
The drybrushing was easy: Chaos Black, Codex Grey, Bleached Bone..
Hope it inspires!