Cinnabar is a mineral. It is the main ore of mercury and is made up of the chemical mercuric sulphide (HgS). Cinnabar exists in two crystal forms. Red cinnabar is used for the pigment vermilion. It is poisonous, and may go black with age. The other crystal form is black, and does not react chemically to anything except strong acids. Cinnabar has a Mohs hardness of 2-2.5. 