conchoidal

Conchoidal describes a fracture pattern seen in brittle, hard materials where breaks propagate along smooth, curved surfaces that resemble the shape of a seashell. The fracture surfaces display a rippled, shell-like appearance and lack a defined crystallographic cleavage. This pattern is most commonly associated with glassy substances and some minerals.

The term derives from concha, the Latin word for shell, reflecting the characteristic curved surfaces. Conchoidal fractures produce irregular, curved planes that can form sharp edges, and the surface often exhibits concentric ripples or ridges created as the fracture front advances.

In geology and materials science, conchoidal fracture is typical of glassy rocks and materials such as obsidian and other volcanic glasses, as well as many forms of man-made glass. Some cryptocrystalline quartz varieties can also show this fracture, though they may display other fracture characteristics in different contexts. The resulting edges are usually very sharp, which has practical implications for toolmaking and cutting applications.

In archaeology and lithic technology, conchoidal fractures explain how sharp flakes are produced through percussion, enabling the fabrication of tools from flint, obsidian, and similar materials. The pattern helps distinguish knapped stones from those with other fracture types and supports interpretations of ancient manufacturing techniques.

Overall, conchoidal fracture is a key diagnostic feature for identifying glassy or highly brittle materials and understanding their mechanical behavior when broken. Related concepts include fracture and cleavage, as well as the broader study of fracture mechanics. Examples of materials exhibiting conchoidal fracture include obsidian, glass, and certain quartz varieties.