Realgar is a mineral composed of arsenic sulfide, with the chemical formula As4S4. It typically appears as a bright red or orange crystalline substance and is often found in hydrothermal veins and hot springs. Realgar is known for its use in traditional Chinese medicine and as a pigment in paints. Due to its arsenic content, realgar can be toxic and poses health risks if not handled properly. It has been historically used in fireworks and as a pest control agent. The mineral is also studied for its potential applications in various fields, including mineralogy and geology.