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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.
All conceivable minerals fit in this scale, since Talc is the softest known mineral and Diamond the hardest. To demonstrate how to use the scale, understand the following example: Suppose a mineral scratches Fluorite, but not Apatite, then it has a hardness between 4 and 5.