a fracture having smooth convexities and concavities like a clamshell. [SEMATECH] Also see chip.

Glass fractures conchoidally. The shock wave produced when the glass is struck fans out from the point of impact and spreads through the glass. The next example I will give has become a cliche' in flintknapping circles, but that is because it perfectly describes conchoidal fracturing. When a pane of glass is hit with a BB from an air rifle, a cone of glass is removed and falls out the other side of the pane. That cone is called a herzian cone. We want to use that cone of force to remove flakes from stone that has the ability to fracture in this manner... conchoidally. The scar from conchoidal fracturing found on the surface of a point is kind of "shell" shaped. DISTAL- The tip end of the point.

breakage like a shell Obsidian

A type of breakage that produces a smooth, curved surface. Conchoidal fracture occurs when a substance has uniform strength in all directions and no pre-existing planes of atomic weakness. This generally occurs in two types of substances: minerals like quartz whose atomic structure consists of equally strong bonds in all directions, and volcanic glass, called obsidian, which has no definitive crystal structure.

Conchoidal fracture describes the way that brittle materials break when they do not follow any natural planes of separation. Materials that break in this way include flint and other fine-grained minerals, as well as most amorphous solids, such as obsidian and other types of glass.