Artificial rocks can be created cheaply and easily if you make your own from Hypertufa.

• Make a batch of rocks by digging several holes in the ground the size and rough shape you want the rocks to be.
• Line the holes roughly with thick polythene, do not worry about the odd crease and fold as this will add texture to your rock surface.
• Mix 2 parts sand, one part cement and two parts coir together in the dry state. You can add cement die, buff or yellow seem to work well.
• Add water and mix in to create a fairly stiff mixture.
• Put the mix (Hypertufa) into the lined holes pushing in into the bottom and sides. You can leave a central hole but make sure there is at least 2″ of mix all around. This makes the rocks lighter and easy to move. You can also strengthen with bent chicken wire.
• Leave for a couple of days until the mix is totally dry then lift out of the hole and peel off the plastic.
• If they are a bit shiny you can rough them up with a wire brush but once in the garden they will soon weather, grow mosses and lichen to look quite natural.

See photos on google
There are other several Hypertufa rock recipes you could use. Some are more useful for covering an old sink or pot to make a hypertufa plant pot.

Variation 1 1 part Portland cement, 1 part sand, 2 parts peat
Variation 2 1 part cement, 1.5 parts sand, 1.5 parts peat
Variation 3 1 part cement, 1 part peat, 1 part perlite, 1 part fiberglass mesh.