Toning on a coin is essentially tarnish and is caused by a chemical reaction between the metal and some chemical or chemical compound. Toning can be natural (e.g., Morgan Dollars that toned after sitting in cloth bags with a bit of sulphur for several decades) or artificial (e.g., coins with wild and unnatural colors and patterns - you see these regularly on eBay).
Wow! You can see every, little fiber in the cotton gloves with that photo quality! What type of camera are you using?! -Brian
Not necessarily, although coins with great natural toning can bring many, many multiples of the book value. On the other hand, a coin with unattractive natural toning may be bring less than book. And a coin with artificial toning can go either way (depending on the knowledge of the buyer - a novice collector may pay a lot for a coin with "pretty" toning even though the toning was painted onto the coin by a coin doctor). Natural toning is not usually PMD, but if a coin is left too long in an environment that causes toning the toning can progress to its final state (a black coin). An artificially-toned coin is usually not PMD (the toning may be removed with acetone) but I suppose sometimes it may be considered PMD.
