That is very much a natural color "toning" if you want to call it that. I call it acidic corrosion which will for all practical purposes give it a details grade as "altered color". This doesn't mean on purpose but in how it was stored. It was most likely in a very old coin board of the 30's/40's. These kept air and other direct elements from the rim and back. The front was exposed to air, moisture, sunlight and most likely cigar smoke, in a humid climate. Ate that sucker right up. It will grade as Uncirculated (MS60 something low) Altered Color(details)