Orpiment
Orpiment is a mineral. Its chemical formula is As2S3. It is an orange to yellow mineral. It has a Mohs hardness of 1.5 to 2 and a specific gravity of 3.46. It melts at 300°C and oxidizes to arsenic trioxide at higher temperatures. It is toxic because it gets a layer of arsenic trioxide on it by oxidation in the air.
Orpiment was used as a pigment because it was one of the only yellow pigments that they could find. It is used to remove hair from hides.