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Cleavage is the tendency of a material to split apart along flat, smooth planes. These are planes of weak atomic bonding within the material. Cleavage is described as perfect, good, fair, poor, distinct or indistinct. Splitting along irregular flat planes is sometimes called 'parting', but this may not be apparent in all specimens of a given mineral. Many minerals have perfect cleavage, and in some cases in more than one direction. Octahedral cleavage in fluorite (calcium fluoride) allows material to be broken into perfect octahedral shapes. This can sometimes be misleading, since the octahedron is also a typical shape of fluorite crystals. Expertise is needed to distinguish cleavage surfaces from true crystal surfaces.