I'm fairly certain this is some sort of
PMD staining. I'm sharing as I found it super interesting and it is now a part of my collection with the nickname of being my "tiger" penny.
I also think it's cool how the staining pattern is on both the obverse and the reverse (obverse staining is sharper though). With all the other stained Pennies I've found, the staining pattern was limited to one side.

