The physical requirements of stone tools tend to prescribe the choice of rocks for their production. Hardness, roughness and impact toughness dictate the ease of knapping as well as the durability of flaked tools, and these rock properties can be measured by mechanical tests described here. Geochemical and petrographic analyses of archaeological and geological samples can also characterise rocks and, under certain circumstances, they enable rock sources to be traced. Examples of how this can be achieved are provided from sites around the world. At Sibudu, KwaZulu-Natal, locally available dolerite and hornfels are the most common rock types employed to make flaked tools. Unfortunately, neither rock lends itself to being sourced, but mechanical tests demonstrate that these rocks have both desirable and undesirable properties for stone-tool manufacture and use. Coarse-grained (rough) dolerite is difficult to knap, but it ensures durable edges, whereas fine-grained hornfels is relatively easy to knap, but the sharp edges on its products are sometimes fragile, require resharpening, and have a tendency to break. Dolerite was thus more suited to tool manufacturing than we initially realised.