Conchoidal fracture and rock cleavage

Fracture and cleavage are terms geologists use for how rocks break. Rock cleavage is when a rock splits along flat, smooth surfaces that are parallel to zones of weak bonding in the crystal structure.

Conchoidal fracture is when a rock breaks with curved, shell-like surfaces instead of flat ones, like broken glass. These fractures are not along any particular planes of weakness and appear more random.

The top image is an example of rock cleavage in sandstone and the bottom two images show conchoidal fracturing of obsidian (left) and flint (right).

Sandstone, Dr Mary Gillham Archive Project, CC BY 2.0; obsidian, Ji-Elle , CC BY-SA 3.0; and flint, Public Domain.