Orpiment | Arsenic Sulphide
Orpiment is an arsenic sulphide that's easy to identify because of its vibrant yellow colour. It's photosensitive hence will fade or develop a white powdery film if exposed to light for long periods of time. Although not considered to be highly toxic, it should be handled as little as possible.
Orpiment is extremely soft and grades just 1½ to 2 on Mohs scale of mineral hardness so it's easy to grind down into a powder. This along with its distinctive yellow colour meant that historically it could be used for inks, dyes and as a pigment in paint and until the 19th century it was one of the very few bright yellow pigments available for use by artists. Orpiment was also traded in ancient Rome and China where it was used as a medicine despite its toxicity.