This drawing shows the cleavage direction with a dashed red line. Although diamond is not fragile or prone to breaking apart, all substances including diamond can fracture or shatter. Due to its particular crystal structure, diamond has certain planes of weakness along which it can be split. Diamond is said to have perfect cleavage in four different directions, meaning it will separate neatly along these lines rather than in a jagged or irregular fashion. This is because the diamond crystal has fewer chemical bonds along the plane of its octahedral face than in other directions.
Diamond cutters take advantage of cleavage to fashion diamonds efficiently.