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A mineralâs hardness is related to the strength of its weakest bond, and minerals may have different hardnesses in different directions. For example, kyanite has a hardness of 5.5 if scratched lengthwise, but a hardness of 6.5 if scratched crosswise (numbers in Mohs scale). Since covalent bonds are stronger than ionic bonds, minerals in which covalent bonds dominate will be harder than those in which ionic bonds are prevalent.
Definition. 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.