My guess is oxidation of the copper in the copper-gold alloy. The mixture of the alloy may have a couple of pockets of copper rich areas that discolored.
Usually, orange or green crud on a gold coin is a good sign because it is a better indicator of original surfaces. I might be totally off my rocker, but I think that the discoloration makes the coin more desireable.
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1928-D MS64
$20 Saint Gaudens Registry Set
<< <i>BTW, sometimes that is the holder and NOT the coin! Make sure you don't penalize the coin if the holder has a problem.
My thought exactly. Looks like schmutz on the holder, the way that it blurs the surface below...
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I agree it does sort of look that way but not so much after looking at the obverse which also appears to have dark areas (i.e. copper rich areas)