Hardness is defined as resistance to scratching and is graded between 1 (softest) and 10 (hardest) using the Mohs scale of mineral hardness.
Diamond has a hardness of 10 (hardest) on this scale ,it is the hardest natural material known. The gemstone hardness in this chart would help reduce damages while mounting a gemstone in jewellery,and endure the occasional hard knock or abrasion.