Ground stone axes were another tool commonly used by the Nok. They were typically made from fine-grained volcanic rock (siliceous rock is also sometimes seen), and may have been used in food preparation. These ax blades tend to be smallish in size, the largest reaching 20 centimeters. Stone balls are found at almost every Nok site and are approximately palm-sized. They were probably used as hammerstones or for roughening the surface of a grinding stone. Not all of them are ball shaped, however, and many have chipping marks all over or at least in one place. These stone balls likely would have served as mobile grinding stones.