What is the blue tint on coins?

It's called toning and is a form of tarnish that occurs when the coin reacts with oxygen (and some pollutants) in air.

Toning is prized by many collectors because it can be quite beautiful and is never the same on any two coins, so don't attempt to remove it.

First answer by JayKay. Last edit by JayKay. Contributor trust: 961 [recommend contributor]. Question popularity: 1 [recommend question]