Probably toning from the environment/circulation, and you could check to see if it attracts to a magnet, as there are 2 varieties, the magnetic nickel one, and the one with silver. This was the transition year for Canada from silver to nickel.
Looks like it may of been buried at one time. But in any case, it is some sort of environmental damage. As noted, check to see if it is silver or nickel.
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