Stone points, commonly called arrowheads, are more accurately termed projectile points in archaeology, encompassing tips for spears, darts, and arrows. Creating these tools required raw material with specific physical properties for a predictable outcome during the shaping process known as knapping. Toolmakers sought extremely hard, fine-grained rocks that allowed for the controlled removal of material to form sharp edges and a strong tip. This explains why only a handful of silica-rich rock types were consistently utilized across different continents and time periods.
The Primary Rocks Chosen for Knapping
The most widely used materials for crafting projectile points fall into three main categories of silica-rich stones. Flint and chert are microcrystalline forms of quartz, meaning their crystals are only visible under a microscope. Flint, often found in nodules within chalk or limestone beds, is generally considered the higher quality, darker-colored variety, though the terms are frequently used interchangeably. Their high durability and reliable fracture pattern made these sedimentary rocks a staple material worldwide.
The second primary material is obsidian, a naturally occurring volcanic glass formed when silica-rich lava cools too rapidly for crystals to grow. Obsidian is amorphous, meaning it lacks an internal crystalline structure, allowing it to fracture into exceptionally thin pieces with edges sharper than a surgical scalpel. While more brittle than flint, its unparalleled sharpness made it a valuable trade commodity, especially in regions with volcanic activity.
Jasper, the third primary material, is an opaque form of chalcedony or chert, characterized by its fine grain and vibrant colors due to iron oxide impurities. This fine-grained texture allows for clean, controlled flaking, and its striking red, yellow, or brown coloration made it a sought-after material for points.
The Geological Key: Understanding Conchoidal Fracture
These specific rocks were chosen due to a unique material property called conchoidal fracture. This fracture describes a shell-like breakage pattern that occurs when force is applied to a brittle, fine-grained, or amorphous material. Unlike rocks with natural cleavage planes, these ideal toolstones lack internal planes of weakness, preventing breakage along flat surfaces.
When a knapper strikes the stone, the force propagates through the material as a shock wave, creating a cone of force that initiates the fracture. This process produces smoothly curved, concave surfaces, resembling the concentric ripples on a mussel shell, from which the term “conchoidal” is derived. A small, rounded swelling called the bulb of percussion is left on the detached flake, providing archaeologists with a clear signature of intentional human modification. The lack of a preferential fracture direction allows the toolmaker to precisely control the impact, ensuring predictable material removal for shaping a sharp point.
Regional and Secondary Stone Sources
When primary stones were unavailable, toolmakers utilized regional and secondary sources. Quartzite, a metamorphic rock derived from sandstone, was sometimes used if it possessed an extremely fine grain and had been highly metamorphosed. Its quality varies widely; while some glassy varieties can be worked well, coarser types often result in a less uniform and more sugary fracture.
Chalcedony, a translucent, waxy form of cryptocrystalline quartz closely related to chert, was widely utilized. It generally knaps cleanly and was a reliable material where it naturally occurred. Petrified wood, formed when organic material is replaced by silica, was another source, provided the resulting structure was uniform enough to fracture predictably. These secondary materials often required greater skill or heat treatment to achieve the flaking control found in high-grade flint or obsidian.