Diamond is the high-pressure form of carbon, written C. It is a native-element mineral and the hardest common mineral on the Mohs scale, hardness 10. That hardness is directional: diamond can scratch every other mineral, but it also has cleavage planes and can chip if struck along them.
Crystals are isometric, most often octahedra, cubes, or rounded forms from resorption in the magma that carried them up. Colorless stones are purest, but natural diamond also occurs brown, yellow, pink, blue, and rarely green or red. Blue color is often linked to boron; yellow often to nitrogen.
This page is only for diamond, not for glass imitations or moissanite.