Conchoidal fracture

A conchoidal fracture is produced when some types of fine-grained mineral, such as obsidian and flint, are broken. These minerals were widely used in the stone age to make sharp tools. They occur also in amorphous solids such as glass.

Conchoidal fractures often result in a broken, curved surface that resembles the rippling, gradual curves of a mussel shell, the Greek word for which conchoid derives. A swelling appears at the point of impact called the bulb of percussion. Shock waves emanating outwards from this point leave their mark on the stone as ripples. Other conchoidal features include small fissures emanating from the bulb of percussion.

They are defined in contrast to the faceted fractures seen in single crystals such as semiconductor wafers and gemstones, and the high-energy fracture surfaces desirable in most structural applications. As conchoidal fractures can be produced only by mechanical impact, rather than frost cracking for example, they can be a useful method of differentiating prehistoric stone tools from natural stones.

Such fractures form the basis of flint knapping, since the shape of the broken surface is controlled only by the stresses applied, and not by some preferred orientation of the material. This property also makes such fractures useful in engineering, since they provide a permanent record of the stress state at the time of failure.

See also: Hertzian cone.