It's sad, but many researchers, dealers and long-time collectors believe that about 80% of all 20th Century silver coins and 98% of 19th Century and earlier silver coins have been cleaned or at the very least, dipped by somebody.  This is done primarily to increase a coin's value.  Today, we also have the "coin doctors" who use a variety of methods to artifially tone a coin to increase it's value, maminly to increase a coin's value.  It seem like someone is always messing with our hobby to make a buck.

Coin doctoring aside, the reasons coins tone are numerous.  If you can think of toning on coins as rust is to steel or oxidation is to aluminum, then it's a bit easier to understand.  Toning can be gorgeous or it can be really ugly, but believe it or not, all coins are susceptible to toning.  The following coin images are divided into two categories; naturally and artificially toned.  Each thumbnail image is linked to larger, slower loading image with the cause of the toning explanied if it is known.

ARTIFICALLY TONED COINS