Ever Seen Blue Tone On State Quarter

For some reason some dish soaps create that effect. I don't mean dipping a coin in dish soaps, but if you've been using some and then handle some coins, many will turn bluish. The thing to remember is toning, AT or NT is created by the outer layer of material of a coin with almost anything in the invironment. Air, moisture, air born contaminates. For example one of the biggest culprits to toning is in areas where Sulfur Rich Coal is burned. Power Stations, steel mills, etc. That Sulfur goes into the air as SO or SO2 and mixes with moisture in the air to form an Acid. As most have heard of Acid Rain and that is what this is. AND if effects coins easily as well as ruining almost anything.
If you have a pool or live near a large pool, note the main thing people put in there is a form of Chlorine. This tends to evaporate into the air and if your coins are near that, that too effects the way they tend to change.
Garbage dumps produce Ammonia Gasses that also effect coins.
And don't forget that Garlic you put on food. Might not effect coins but sure makes for bad breath.