Toned or toning| are terms usually used to describe silver coins, and essentially mean tarnished. Using the word tarnished, however, would indicate something negative and might suggest the coin should be cleaned. Toning is considered a positive attribute, not a detraction, and is usually not something that should be corrected.
Toning is oxidation and results from exposure to the air. "Light toning" is just a slight darkening. "Dark toning" could mean the coin that is entirely dark black. The longer a coin is exposed to the air, the darker it will become.
Natural, uncleaned toning will be darker where the coin’s surface has had more exposure to air and lighter where the surface has had less exposure to air. Coins with light toning may have even lighter "tone shadows" in the recesses around the coins features, where those features have restricted the flow of air. As a coin darkens, these "tone shadows" will disappear
Sometimes toning can be colorful and sometimes very attractive and increase the value of a coin. Attractive toning variations include golden toning, rose toning, blue toning, iridescent toning, or rainbow toning. Attractive toning can be faked and detecting well-done fake toning requires some expertise.
The highly desired “old cabinet toning” is attractive toning that has developed since the coin was originally cleaned, perhaps many years ago.
Silver coins often have “highlighting” dark toned fields and brighter raised features and inscriptions. Since the raised features actually have more exposure to air, this is not natural. Fakes and replicas often have this type of toning (sometimes created with paint or shoe polish) because it looks the way an untrained eye expects an old coin to look. Genuine ancient coins may also have “highlighting” toning if they have been partially cleaned to create it; or if they are frequently handled, rubbing the toning off the highpoints.
Sometimes toning can be uneven, mottled, or splotchy, and unattractive. Unattractive toning can decrease the value of a coin. Uneven toning can be natural. If a coin is encrusted with deposits, areas that are encrusted and not exposed to the air will not tone. Untoned areas may be where encrustation was removed. In a hoard, some parts of a coin may be exposed to air while other parts are covered by the vessel or another coin. Uneven toning can also result from a failed cleaning attempt. Cleaning to remove toning should only be considered if uneven toning makes it difficult to see or enjoy the features of the coin. Cleaning should only be performed by someone with the necessary expertise.