Hardness is measured by the resistance which a smooth surface offers to abrasion. The degree of hardness is determined by observing the comparative ease or difficulty which which one mineral is scratched by another.
Table showing Mohs' relative hardness scale. Mohs' original hardness values are highlighted in yellow. The selection of mineral species for hardness standards is based on the common availability of the mineral. An interesting observation is the fact that each Dana class (except for organic class) of mineral is represented by at least one example.
Determinative Mineral Tables
Graph showing the distribution of hardness values by Dana mineral classification.
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Amethyst Mineral Gallery has a nice explanation of mineral hardness.