to make rocks: carve the desired shape out of foam, then cover with black acrylic paint (above). Next buy some spackling paste, and spread a thin layer over all or part of the rock (below). finally, cover in a light coat of black paint again and sort of wipe it off as you go with a paper towel. tada!... a rock.
these silly things are the base coat for a close up of a rock. it is a hard piece of foam, with sandpaper slashes and painted with acrylic paint. soon to be covered with some sand and stuff.