One clue to a mineral's identity is the way a sample breaks off a larger piece of mineral. If the mineral breaks to form fairly flat and smooth surfaces - planes of weakness in the crystal structure - geologists call it "cleavage." Mica and graphite have excellent cleavage because their atoms have strong bonds with each other within crystal planes, but only weak bonds between the planes. In other minerals, the atomic bonds have approximately the same strength in all directions, so they do not break into flat pieces when struck by a hammer. Scientists call this "fracture," and they use several adjectives to describe it. A "conchoidal" fracture has smooth, curved surfaces, while a "hackly" fracture is jagged, with sharp edges. Fracture may also be fibrous, splintery, or irregular.