Orpiment is arsenic sulfide, and an important source of the element arsenic. It occurs rarely in crystals, but generally as yellow grains and masses, almost always associated with another arsenic sulfide (with a bright red color) known as realgar. Both sulfides were used in ancient times as pigments and to some extent are still used in this application. The orpiment crystals from Quiruvilca, Peru are exceptional in terms of size and perfection.