Patina
Patina ( pə-TEE-nə, or PAT-ih-nə) is a thin layer that variously forms on the surface of copper, brass, bronze, and similar metals and alloys (tarnish produced by oxidation or other chemical processes), or certain stones and wooden furniture (sheen produced by age, wear, and polishing), or any similar acquired change of a surface through age and exposure. Additionally, the term is used to describe the aging of high-quality leather.