Toning can be considered a form of environmental damage, pleasing to some and not pleasing to others. It is what happens to a coin after being exposed to chemicals in the environment. Old time collectors used to dust their albums or folders with sulfur which over time toned coins. While some toning is artificial, others can take on natural toning simply from storage. There are many wonderful examples of proof coins sealed in mint cellophane with great toning and I dare say they were not exposed to salt water.