The hardness of a mineral is a measure of its resistance to scratching. The Mohs scale is a standard scale for measuring the hardness of minerals, with a scale from 1 to 10, with 1 being the softest and 10 being the hardest.

A mineral that can scratch a penny but not glass has a hardness of between 3 and 5. Some examples of minerals with a hardness of 3 include calcite and fluorite, while minerals with a hardness of 5 include apatite and magnetite.