Cinnabar. Credit: Mineral Information Institute.

Cinnabar is a heavy, reddish mineral form of mercury (II) sulfide, HgS, that is the principal ore of mercury. Cinnabar occurs in both crystallized and massive granular forms. It occurs in the rhombohedral system as hexagonal (six-sided) crystals, often twinned, and varies from perfectly opaque to almost transparent. It is found in hydrothermal veins and volcanic deposits, notably in Spain, Italy, Peru, and California. It is reduced to mercury by roasting. Hardness 2–2.5, relative denisty 8.0–8.2. Hepatic cinnabar, so called from its liver-brown color, is a variety containing a little carbon. A black cubic form of mercury (II) sulfide, metacinnabar, also occurs.

In the form of red mercuric sulfide, cinnabar is used as a pigment under the name vermillion.