That would be a very tiny sticker.
A drop of clear paint or varnish would protect the surface so it wouldn't change color while the rest of the coin circulates. When the paint/varnish chips off, you're left with a circle of untoned coin.
A drop of clear paint or varnish would protect the surface so it wouldn't change color while the rest of the coin circulates. When the paint/varnish chips off, you're left with a circle of untoned coin.



















