Carbon is both the hardest and softest element occurring in two different forms as graphite and diamond. A single crystal of diamond scores the absolute maximum value on the Knoop hardness scale of 90. Based on the somewhat less informative abrasive hardness scale of Mohs, diamond has a hardness of 10. Graphite is an extremely soft material with a Mohs hardness of only 0.5 and a Knoop hardness of 0.12.