Realgar is red arsenic sulfide, usually described as orange red to orange yellow. Realgar gets its name from the Arabic words for “powder of the mine” because it is formed from volcanic vapors and is very unstable, meaning it eventually becomes a powder. Exposure to light over time will cause it to alter to a different mineral and eventually to a colored powder. Specimens should be stored in dark areas and only exposed to light for brief periods of time in order to preserve them as long as possible.

In the past realgar was a common pigment for red paints and dyes. Many of these works of art now have a yellow or orange hue where once the color must have been an original red.