Cryptocrystalline rocks with high silica (SiO2) content (e. g., flint, chert, jasper, silicified shale, fine-grained quartzite, fine grained basalt, and obsidian) fracture conchoidally unless the mass is flawed. The term con-chodial means ‘shell-like’ because the bulbar surface created by the fracture is reminiscent of a shell. This is because a cone is created when a hard percussor such as a hammer stone makes contact with the surface of the mass or core. This cone type fracture is called a Hertzian cone. The other major fracture initiation type in chipped stone tool manufacture is the bending fracture created by using a billet, punch, or pressure tool. Conchodial and bending fractures can be controlled by an experienced flintknapper and used to one’s advantage. Granular or coarse rocks such as quartzite and rocks with large crystals, such as granite and diorite, are more resistant to fracture and were often selected for hammer-stones, grinding stones, and ground stone axes because of that very quality. Different stones were selected and used for different qualities, and different methods of reduction were used in shaping the stones into tool forms.
The methods of reducing siliceous and crystalline materials included the uses of hard hammers, soft hammers (billets), punches, pressure, pecking stones, and grinding stones. The qualities of a rock were used to the advantage of the ancient stoneworker. Homogenous rocks with high silica such as chert, flint, and obsidian fracture in highly predictable ways. Coarse rocks with granular or crystalline structure such as quartzite or granites, or rocks with cleavage planes such as silicified wood or laminated shale, fracture differently. How rocks fracture or resist fracture is one of the main topics addressed.
World History
