Over the past 15 years an industry has developed creating rock like structures for both interior and exterior applications. These larger structures generally start out with blocks of expanded polystyrene that are field cut to the general shape of the project. The foam is then covered with wire which will hold the concrete in place.
Next, the concrete is applied either by hand and trowel or sprayed under pressure as shotcrete, depending upon the size of the project. As it sets up, the final shaping of the concrete is done by hand. After it dries, multiple coats of color and sealants are applied to make the final product. If the project is done correctly, it looks like it was created my mother nature.