Hardness
hard·ness | ˈhärd-nəs
noun
The resistance of a substance to being scratched
Hardness refers to the amount of resistance a substance offers to being scratched. The hardest known mineral is diamond, which is 5-15 times harder than the next hardest, sapphire. There are several ways by which gemstones and minerals are measured for hardness, depending on the testing methods utilized. Additionally, the different directions on the surface of a stone will offer variable resistance to scratches and abrasion so must be rated carefully. The hardest direction, for example, on a diamond is the parallel surface of the face of the octahedron. On average, using the Mohs scale, diamonds have a hardness of 10, the highest of naturally occurring elements.