Conchoïdal

Conchoïdal, also known as conchoidal fracture, describes a type of fracture surface produced when a brittle material breaks without any natural cleavage planes. The resulting surface is smooth, curved, and shell-like, often displaying a pattern of ripples or concentric ridges that reflect successive crack fronts. This fracture mode occurs in materials that lack predictable planes of weakness, such as glassy or vitreous substances.

Common examples include obsidian and other volcanic glasses, flint and chert, and many forms of quartz. In these materials, fractures propagate in curved trajectories, producing sharp, blade-like edges that are highly valued in tool production and in the handling of brittle substances. By contrast, minerals with true cleavage tend to split along flat, well-defined planes rather than forming conchoidal surfaces.

Conchoïdal fracture is of particular interest in archaeology and lithic technology because it explains how early humans produced sharp flakes and blades through controlled removal from a core. The characteristic curved, rippled surfaces help distinguish conchoidal fractures from other fracture types in prehistoric artifacts and modern glass.

In geology and materials science, conchoidal fracture indicates a lack of crystalline cleavage and provides clues about the material’s internal structure, brittleness, and the mechanics of crack propagation under stress.