Greg M. writes: I have an 1891 Morgan Dollar that has a distinct discoloration. I have been told that this could be part of a certain collection from which all coins were discolored by some means or another but have been unable to find any specifics. Thanks for your time.
Toning, or to use another term, patina, is the result of coin metal combining with oxygen in the air. The depth of the oxidation and the type of chemicals that might have been in the air in the presence of the coins, determine what the patina will show. Silver coins kept in the same environment for a long period of time will most likely show similar patterns of patina.
A good example are coins that have been kept in cardboard albums for years. Albums like this contain high quantities of sulfur that was used in its manufacture. Sulfur combines easily with coinage metals like silver, copper or nickel and is capable of creating a multicolored patina on each coin.
Experienced numismatists might give an educated guess on what caused a particular type of toning. Identification by patina only works if the coins are very rare and can be traced back to an original auction sale.