I have no idea what the cent was exposed to but, whatever it was it wasn't very even. Possibly was in someone's pocket while exercising. Sweat can have that effect. It looks like the obverse was against some type of fabric.
Someone must likely baked the coin,people advertise tons of them as beautiful tonersand get a pretty profit.
After all it's a Millennium Coin--a forbearer of the deteriorating value of what was once bright and shiny (in jest, of course)!
Ever since 1982, the Lincoln cent has become a standard for material cheapness. Hopefully, the U.S. Mint stops producing the "zincolns" for circulation and mints Lincoln cents for collectors only, changing the composition back to the original bronze. Sorry I got off the topic somewhat.
Are you referring to the vertical lines that are quite noticeable on the obverse? I know I've seen lines like those on many cents and wondered what they are and/or why they are there.
Offer it on eBay and call it "monster toning." $299 plus shipping. Say you've looked at 1000's of cents, and have never seen one like it, ever before, in your entire life. Add "I mean it, I mean it, I mean it, I mean it!" Let us know how the bidding goes.