http://www.ebay.com/itm/1964-Jeffer...27?pt=Coins_US_Individual&hash=item1c30ee71af This would be an amazing coin if it weren't for all of those black spots.
...lol...looks like who ever broke that coin out of its proof set had a cold or bad allergy and sneezed all up on it before putting it away :smile
dust, a speck of some other foreign object, sulphur in the environment and various other things. Once that combines with moisture, the dirty deed begins. I think moisture alone will do that also.