Several manufacturers have found ways to create realistic-looking rock out of man-made materials. Typically made from a special formulation of cement, faux rocks may be molded after actual rock formations found in nature, or they may be hand-sculpted by a faux rock expert onsite.
While faux rock may not fool everyone, it has advantages, including (usually) lower cost. It also tends to be more chemical-resistant (especially when compared to sandstone or limestone), and can produce more predictable results, as compared to the use of natural stone.