Once you know how and where you want to use a good sized rock or boulder in the landscape, you can make your own. There are latex forms you can use for molds or you can build your own form. Rocks and boulders can be fabricated out of cement, resin, plaster of Paris, and other materials. Here is some information about building cement rocks and boulders.
You will find it easier to build a large fake rock if you sketch it out on paper first. It can change as you construct it, but with a basic plan to follow you will have more control over the final results.
Next you will need to construct a skeletal framework or a basic form which will define the shape of the concrete.
For a small rock you can use a purchased prefabricated form to pour full of cement. Or you can make an impression of a real rock by pouring your own contractor’s sealant or latex material over the rock and pealing it off to give you a form to fill with concrete.
Small or medium rocks can also be built on forms you can sculpt with Styrofoam or foam rubber. This shape will need to be covered with a wire mesh so the cement has a texture to adhere to firmly.
For a large boulder try building a rough framework out of wood and rebar. Cover the form with a mesh such as hardware cloth or a small gauge chicken wire. Hold your pieces together with screws, nails and metal wire, sculpting and folding the surface of the mesh to create your rock shape.
Mix up your cement as a combination of Portland cement, water and sand with little water so the consistency is thick and easily molded with your hands. Adding perlite or vermiculite can make your mixture a little lighter and can introduce some texture to your rock finish. You can also add powdered colorants at this stage for the overall tint of your rock. There are many ways to texture and color cement rocks and bounders.
Apply the first coat of cement expecting a lot of the cement will filter through the mesh and be lost. You can do this with plastic sheeting beneath so you can re-use the cement for additional coats. As you continue to add cement in layers you will end up covering the frame completely. Use enough cement to provide a coating that is thick enough to cover the materials beneath and provide a shell that is dense enough to support any weight that might be appropriate for its use. The final layer should be used to make your rock look real. Sculpt your textural effects into the moist concrete surface to get the effects you want. You can also press materials like slabs of rock or textured cardboard into the surface to create surface effects.
When your rock or boulder is dry, paint it with spray paint or a brush. Adding cracks, lines and veining will make it more realistic.
For more info:
Dress up the landscape with exterior design
Recycle materials in the landscape