Conchoidal fracture describes the way that brittle materials break or fracture when they do not follow any natural planes of separation.

Why do conchoidal fractures occur?

“Conchoidal fracture” is named so because the curved lines on the fracture surface resemble the rippling growth lines on the shells of clams or conchs. The sample above is sandstone with abundant phosphatic brachiopod (Lingulata) shells from the Ordovician of Estonia.

What rocks have a Conchoidal fracture?

Conchoidal fracture

It often occurs in amorphous or fine-grained minerals such as flint, opal or obsidian, but may also occur in crystalline minerals such as quartz.