Realgar
Realgar is a mineral. Its chemical formula is As4S4. It is a soft (Mohs hardness 1.5 to 2) red mineral. It is an ore of arsenic. It changes into a yellow powder when exposed to light. This yellow powder is called pararealgar. It melts at 320°C and burns with a blue flame, making arsenic and sulfur fumes. It was used to make a white color in fireworks before powdered metals were used. It is still used in some crackling firecrackers. It was used as a red pigment and an item of trade. It is used as a medicine in China. Its uses are similar to orpiment, a similar mineral.