The ability of a diamond or colored gemstone to resist scratching. A diamond is by far the hardest specimen in nature. See also Mohs Scale. Nothing else in nature even comes close. Corundum (as in sapphire and ruby) is second on the scale, but they are far behind.
This is not to be confused with a diamond’s Toughness, though, which is the measure of a gem’s ability to resist breaking from a blow.