Copper Point (FOUS 98266) - Public Domain image, National Parks Gallery

This copper projectile point was found during Fort Union’s 1988 archeological excavations. Notice anything unusual? The green coloring you see is not a decorative element but rather copper carbonate, the presently stable byproduct of a once corrosive chemical reaction caused by contact with moisture and salts while covered by dirt. That chemical reaction can also be triggered by exposure to high humidity and air pollution, which is why outdoor statues such as the Statue of Liberty and older pennies, which are made of copper and copper alloys, sometimes have similar green coloration.