patina

from The Century Dictionary.

• noun A bowl; a patella.
• noun An incrustation which forms on bronze after a certain amount of exposure to the weather, or after burial beneath the ground.
• noun By extension, the surface-texture or -color which other works of decorative art, as a wooden cabinet or the like, gain through the action of time.
• noun The surface, produced partly by accretion, partly by discoloration and the effects of acid in the soil, given to marble by long inhumation.
• noun [capitalized] [NL.] In conch., a genus of gastropods.

from the GNU version of the Collaborative International Dictionary of English.

• noun A dish or plate of metal or earthenware; a patella.
• noun (Fine Arts) The color or incrustation which age gives to works of art; especially, the green rust which covers ancient bronzes, coins, and medals.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun originally A paten, flat type of dish
• noun The color or incrustation which age and wear give to (mainly metallic) objects; especially, the green rust which covers works of art such as ancient bronzes, coins and medals.
• noun A green colour, tinted with grey, like that of bronze patina.
• noun figuratively A gloss or superficial layer.
• adjective Of a green colour, tinted with grey, like that of bronze patina.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a fine coating of oxide on the surface of a metal