Hello Dana! Depending upon what kind of rocks you want to paint, try the following and see what you get;
Using a suitable colour (try Cobalt Blue and Burnt Sienna to achive a neutral grey), paint in the rock as a solid colour, then carefully and lightly scrape over the grey with the flat edge of a Palette Knife or (with great care!) an old razor blade.
Try to decide where you want the light to come from, and lightly scrape on this part to get a better 3 dimentional effect, leaving the darker shadow area as solid.
You'll find that the texture of the paper will affect this, but a rough watercolour paper (try it on SAA Practice Watercolour Paper - Rough) will give a better result.
You can practice with different pressure and different colours, but this should give you a good starting point, to inspire you to have a go.
PS Let me know how you get on.