Tenacity, or toughness, is the cohesiveness of a mineral. It is the resistance the mineral or gemstone offers to breakage. Tenacity and hardness, though not necessarily related, are factors that determine the durability of a gemstone.
Despite their incredible hardness (10), diamonds are relatively brittle because of their excellent cleavage. Nephrite Jade has a Mohs hardness of only 6-6.5, but is not easily broken.
Tenacity is given in degrees of toughness:
- Exceptional, as in Jadeite and Nephrite
- Excellent, as in Sapphire
- Good, as in Quartz and Spinel
- Fair, as in Tourmaline
- Poor, as in Feldspar and Topaz