CoachThe hardness of a mineral is how well it resists being scratched: a mineral scratches every softer one and is scratched by every harder one. The Mohs scale ranks hardness from 1 to 10 with ten reference minerals, from talc (1), the softest, to diamond (10), the hardest.
Examples
Example 15.6 (Quartz or calcite?)
Two white minerals look alike. The first one is scratched by a copper coin: its hardness is less than 3.5; it is calcite (3). The second one is not scratched by a steel nail and scratches glass: its hardness is more than 6.5; among the reference minerals, it is quartz (7).