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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.
He chose ten common minerals (minerals that anyone interested in minerals is likely to have in their collections) and used them to set up a 'Hardness Scale'! He assigned each of the ten minerals a hardness number from 1 to 10, with 1 being the softest and 10 being the hardest!