Coin history
What coin toning is
Toning happens when a coin's surface reacts slowly with air and the materials around it. It can produce soft rainbow colors or deep shades of gold, blue and brown.
Original toning that developed naturally can be attractive to collectors. Artificial toning is a different matter, and collectors are generally wary of coins whose color was added on purpose.
Toning is a reason to keep coins in holders that do not release harmful chemicals, since some materials can damage a coin's surface.