Since I'm running a theme week on my own journal, I decided to continue the theme here -- that being words beginning with O, of which this has long be a favorite. As a pigment, orpiment was the most commonly used bright yellow until cadmium yellow was developed, despite being toxic. As a mineral, it's quite pretty. The name has been used since the late 14th century, from French, from Latin auripigmentum, gold pigment, from aurum, gold + pigmentum, pigment -- making it a case of exactly what it says on the tin.
The forger was quite good -- she even knew to keep the orpiment from touching the verdigris to avoid chemical reactions.