So this was the only applicable thread I thought I could find when looking for something on toned hammered coins - certainly nothing compared to the thread "Let's See YourToned English Milled Silver!". I picked up this James I Halfgroat because it is a tougher mintmark, and was a very decent price. The toning on the reverse is intense, and to be honest reminded me of people who artificially chemically tone US Morgan Dollars. But I can not image for the life of me, someone going through the trouble on a random J1 Halfgroat - so I am assuming it is for real. My guess would be that someone had this obverse up for a long time on a piece of paper which is not archival safe, and the chemicals just toned the reverse, accelerated by humidity if it was not kept well? If it was in something like a sleeve, I would expect both sides would be equally toned.