Toughness is a gem's resistance to chipping and breaking. It differs from hardness, which is resistance to scratching.
In more detail
Corundum has excellent toughness, amethyst good, and emerald fair to good. Diamond is good along its cleavage directions and exceptional in others. Moissanite showed no cleavage in GIA's tests, but its girdle can chip under hard wear. Fractures and inclusions can lower the toughness of a single stone.
Related terms
- Hardness (Mohs): Hardness is a gem's resistance to scratching. Gemologists rank it on the Mohs scale, on which diamond is 10.
- Cleavage: 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.
- 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.