Toning is usually a patina on the surface of the coin which adds color other than the color of the metal. These colors can be vibrant blue, green, red, gold and yellow hues. Or they can be ugly brown, or black depending on the environmental conditions which caused the toning.
Toning is caused by oxidizing reactions on the surface of the coin. Depending on the type of chemical in the environment depends on which color of toning you will get. Some coins are artificially toned (AT) by either heat or chemicals. If this is found to be the case, AT'ed coins are no different from cleaned coins when it comes to price... AT is damage.
Here is a sample of a Naturally toned Proof coin
Here is an example of textile toning from mint bag storage.
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