The Statue of Liberty’s exterior is made of copper, and it turned that shade of green because of oxidation. Copper is a noble metal, which means that it does not react readily with other substances. At the Statue’s unveiling, in 1886, it was brown, like a penny. By 1906, oxidation had covered it with a green patina. The thin layer of oxidation that covers copper can preserve the metal for centuries, even millennia, as shown by objects from the ancient world.

Source: The New Yorker