Cleavage is a mineral's tendency to split along flat planes in its crystal structure. A hard knock in a cleavage direction can split a stone.
In more detail
Diamond has cleavage, so it can split even though it is the hardest gem. Corundum has no cleavage. GIA's researchers broke a moissanite cube with a hammer: it showed curved, shell-like fractures and no cleavage.
Related terms
- Toughness: Toughness is a gem's resistance to chipping and breaking. It differs from hardness, which is resistance to scratching.
- Hardness (Mohs): Hardness is a gem's resistance to scratching. Gemologists rank it on the Mohs scale, on which diamond is 10.
- Fracture: Fracture is the way a gem breaks when it does not split along a cleavage plane. Conchoidal fracture leaves curved, shell-like surfaces.
- Durability: Durability is a gem's ability to resist scratching, breaking, and damage from heat, light or chemicals. It has three parts: hardness, toughness and stability.
- Crystal system: A crystal system groups crystals by the symmetry of their atomic structure. Gemologists use seven: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic.