A mineral having composition As₂S₃ and crystallising in the monoclinic system. Crystals are small, tabular, and rarely distinct (see illus.); the mineral occurs more commonly in foliated or columnar masses. There is one perfect cleavage yielding flexible folia which distinguishes it from other minerals similar in appearance. The hardness is 1.5–2 (Mohs scale) and the specific gravity is 3.49. The lustre is resinous and pearly on the cleavage surface; the colour is lemon yellow. Orpiment is associated with realgar and stibnite in veins of lead, silver, and gold ores. It is found in Romania, Peru, Japan, and Russia. In the United States it occurs at Mercer, Utah; Manhattan, Nevada; and in deposits from geyser waters in Yellowstone National Park.