I assume you are tallking about small rocks and boulders for a model railway/Warhammer type of scenery?

All you need to do it to scrunch up small oval shapes of newspaper or aluminium foil for the boulders and irregular shapes for rocks. Cover these with paper strips. For smooth rocks paint as they are, but for rough peices cover with a thin layer of pulped paper mache. Coat in emulsion paint then paint with poster or acylic paint. For bigger rock pieces, use could cover chunks of polystyrene.

Have fun making them!