Patina
A surface layer and colour that forms on metal, wood or stone with age and exposure.
Also: weathering
Left in the open air, copper and bronze slowly turn from bright orange-brown to dark brown and then to a soft green. That green skin is a patina, and it protects the metal beneath. The Statue of Liberty and many church roofs owe their colour to it. Sculptors also create patinas quickly with chemicals and heat. The word is used more broadly for the mellow surface that old wood, leather, stone or silver gains through years of handling and polishing, which collectors value and are careful not to clean away.