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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.
The following minerals are categorized according to Hardness. The following chart indicates how you might test a mineral for hardness according to Friedrich Moh's scale. 10 is the hardest, and 1 is the softest mineral. (Links are to photographs of the minerals.). The number at the left indicates the hardness of the mineral. The next column has the object of known hardness that will scratch the representative mineral listed in the last column. The mineral name is linked to a picture of the mineral. Move your mouse over each name to view the image in a new window.