Concrete as a building material is good for creating more than just slabs and walls. Because concrete is pourable, you can use it to create stones for use as veneer, covering an existing surface to create the appearance of a stone wall built with naturally occurring stones. These "cultured stones" appear only as real as the mold makes them. With silicone rubber and a stone as a model, you can create a mold capable of producing realistic looking cultured stones, expanding your building options greatly, and using nothing more than concrete as the building material.
build a frame in which to create the mold, using wooden planks. Measure the size of the actual stone that you are using as a model for the cultured stone mold using a measuring tape. Cut the planks to create a box large enough to hold your measured stone with ½ inch of space surrounding the stone on all sides, and with sides 2½ inches higher than the stone. Use wood screws to connect the frame sides and bottom at the joints. Place a strip of masking tape along all joints on the inside of the frame to prevent leakage of the rubber mold-making material.
Place the frame onto a flat work surface. Spread a layer of clay inside the frame covering about two inches of the frame bottom, to serve as a bed to stabilize the stone as you create the mold. Spray the clay and all surfaces within the frame with mold release agent. The release agent ensures that the rubber will not stick to the sprayed surfaces.
Place the stone into the clay, centered inside the frame. Make sure the side you wish to recreate is facing upwards. Push the stone slightly into the clay, about ¼ inch, and then spray the exposed surfaces of the stone with the release agent.