In mineralogy, tenacity is a mineral's behavior when deformed or broken.
Common terms
Brittle:
The mineral breaks or powders easily. Most ionic-bonded minerals are brittle.
Malleable:
The mineral may be pounded out into thin sheets. Metallic-bonded minerals may be malleable.
Ductile:
The mineral may be drawn into a wire. Obviously not easy to test. Malleable materials also may be ductile.
Sectile:
May be cut smoothly with a knife. Relatively few minerals are sectile.
Elastic:
If bent, will spring back to its original position when the stress is released.
Plasticity:
If bent, will not spring back to its original position when the stress is released. It stays bent. In contrast, flexibility is the ability of a material to deform elastically and return to its original shape when the applied stress is removed.