Diamond is the hardest natural material known, where 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.[14] Diamond's hardness has been known since antiquity, and is the source of its name.
Diamond hardness depends on its purity, crystalline perfection and orientation: hardness is higher for flawless, pure crystals oriented to the ⟨111⟩ direction (along the longest diagonal of the cubic diamond lattice).[15] Therefore, whereas it might be possible to scratch some diamonds with other materials, such as boron nitride, the hardest diamonds can only be scratched by other diamonds. In particular, nanocrystalline diamond aggregates were measured to be harder than any large single crystal diamond. Those aggregates are produced by high-pressure high-temperature treatment of graphite or fullerite (C60).[16]
The hardness of diamond contributes to its suitability as a gemstone. Because it can only be scratched by other diamonds, it maintains its polish extremely well. Unlike many other gems, it is well-suited to daily wear because of its resistance to scratching
