Ruby, a variety of the mineral corundum, has a hardness of 9 on
the Mohs scale of mineral hardness.
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The Mohs scale is used to determine the relative hardness of
minerals.
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Friedrich Mohs is famous for creating the Mohs scale of mineral
hardness. He formulated a scale of one to ten and assigned each
mineral a value. This eventually became the basis for the Mohs
scale.