I think it is toneing. Is just a nice word for tarnish. Tarnish is caused by the metal being in a bad environment for too long of a time. So the next logical question is what was in the environment that caused the tarnish. The short answer is I do not know.
I could speculate,however, and this works into my point of third hand research, that the coin was in contact with a corduroy type material for a long period and also exposed to an agent that caused the toneing. As parts of the coin was touching a material and parts not, the colors are different according to the parts touching and the parts not touching.
Edit to ask if I am spelling toneing incorrectly?