Realgar, Arsenic Sulphide,
Formula AsS.

Crystal system: Monoclinic.
Hardness: 1.5 to 2
Density: 3.6 gms/cm³
Colour: Red or orange-red.
Streak: Orange-yellow.

Realgar is usually bright red when fresh. However, particularly when exposed to light, it oxidises and gradually changes to yellow arsenolite and orpiment.(both having the formula As₂O₃}. To retain a bright red colour the mineral must be kept in a closed container in the dark. Realgar is found in low-temperature thermal veins, usually in association with other arsenic (As) or antimony (Sb) minerals. Realgar and orpiment were originally used as pigments but this practice was stopped because they are poisonous.

The specimen shown displays the lovely red colour of realgar and comes from Shimen, Hunan Province, China.