"verdigris"

(noun) - Verdigris is a green or bluish-green pigment. Its name comes from the Old French vert de Grece, which literally meant "green of Greece". It was used as the primary green pigment up until around the 18th century. Think of the way copper changes color, as with an old penny - that weird looking green stuff is verdigris. (By the way, it is a poisonous chemical compound. If you come across a small child attempting to insert such a penny into a nostril, you may consider frightening him or her into sensibility with that "poisonous" part.)