Cleavage is the tendency of a mineral to break along flat, repeated planes where bonds are weaker. Those planes produce predictable, reflective surfaces that show up again and again on new breaks. Cleavage differs from random breakage because it repeats at consistent angles (for example, mica splitting into sheets or calcite breaking into rhombs). Collectors use cleavage to identify common minerals and to predict how a piece will split during trimming.
When you suspect Cleavage, rotate the specimen and check whether breaks repeat at the same angles across multiple edges. Count cleavage directions (one, two, three) and note whether surfaces are smooth and shiny or stepped and imperfect. Don’t confuse cleavage with flat saw cuts or with fracture faces in the surrounding rock—fresh breaks through the mineral grain are the best check. Cleavage notes are also practical for lapidary decisions because strong cleavage can cause unexpected splitting.
Cleavage vs. random breakage Cleavage repeats at the same angles across multiple breaks. Random breakage does not repeat consistently.
Cleavage vs. a flat cut surface Saw marks can look like cleavage. Fresh natural breaks are the reliable indicator.
Cleavage vs. bedding surfaces in the host rock Bedding is a rock-layer surface. Cleavage is inside the mineral and repeats in the grain or crystal.
Cleavage vs. joint faces Joints are fractures in rock units. Cleavage is a mineral property and shows up on repeated mineral breaks.