hard·ness | ˈhärd-nəs
The resistance of a substance to being scratched
Experiment results have proven that diamonds are approximately five to fifteen times as hard as corundum (sapphire), the next hardest mineral. The different numbers come from the different testing methods, and the fact that multiple directions on a given stone surface show considerable variation in resistance to abrasion. The hardest direction in a diamond is parallel to the faces of the octahedron.