While real rocks are certainly cheap and available, they're much too heavy for most stage productions. Lightweight, homemade rocks are easy to move between scenes and won't pose an injury risk to actors if they fall or roll. Make your own rocks from inexpensive materials that you can easily move and custom-build to fit the shape, size and colour needs of your production.
Create a frame from chicken wire and duct tape for a large rock by forming the wire into a large, irregular and hollow ball. Trim away or fold in any loose, sharp wire ends. For a small rock, use a lump of oil-based clay.
Tear strips of newspaper measuring about two inches by five for a large rock, or one inch by three for a small rock (these are rough estimates; adjust the sizes of the pieces as you go to get good coverage and avoid bunched-up areas of too much paper).
Dip the strips of paper in wheat paste and wrap them around the frame. Let the first layer dry (this takes about three hours, depending on moisture levels where you work). Add two more layers and let this dry. Continue doing two layers at a time until you've done about eight total. Let the piece dry completely before you continue.
Saw the rock shape in half, if using a clay frame. Scoop out the clay. Glue the pieces back together with wood glue.
Paint the rock with matt grey paint. If you like, add some marbling texture by applying a lighter grey paint using a scrunched-up magazine page. Alternately, spray with granite texture spray.
Break off chunks of polystyrene of the size of rock you want. Carve shapes as desired with a craft knife or saw, or sand for smooth rocks. If need be, glue together pieces of foam with wood glue for larger rocks. Almost any shape will look good in the final product, provided you cut off areas that have an obviously machine-manufactured straight edge, letter imprints or other obviously unnatural shapes.
Coat the polystyrene with paint primer. Let dry according to the primer's instructions.
Paint the polystyrene with a layer of matt grey, brown or beige paint, or spray with a granite texturing spray.