Toning is a chemical process that occurs on coins that is caused by oxygen in the air acting on the metal or a coin's exposure to sulfur. The process occurs naturally, but can also be artificially implemented to a coin.
Some collectors seek after toned coins, as they are often considered beautiful, while others disregard them. The appearance of a toned coin may vary, and not all tonings are considered beautiful to collectors. When copper tones, it often becomes either green, blue-green, red-brown, brown, or black. Gold over time can eventually tone to an orange color. Silver tones differently than most other metals. The first tone color is yellow, and it then progresses to magenta, then to cyan, and finally black. Other metals, such as aluminum and nickel usually do not appear much different than originally did after the toning process.