Realgar, also known as ruby sulphur, is a reddish-orange mineral that has been used for thousands of years in various applications. It is primarily found in volcanic hot springs and has a distinctive arsenic smell when heated or crushed. Some synonyms for realgar include ruby sulphur, ruby orpiment, red arsenic, red orpiment, and king's yellow. Realgar has been traditionally used as a pigment in art, as well as in Chinese medicine to treat various ailments. However, it is important to note that realgar is toxic and should not be ingested or inhaled without proper precautions and medical supervision.