Natural vs Artificial Toning on Coins. What you need to know.
Toning on coins can add tremendous value, especially if the color is bright and luminescent. Coins tone due to chemical reactions and unfortunately, this process is often sped up by unscrupulous sellers looking to take advantage of that market premium. You see this issue more frequently on common coins, such as modern commemoratives, Silver Eagles, and other low value numismatic material. That's not to suggest that artificial toning does not also happen on rare coins as well. It most certainly does.
Learn how chemistry impacts your coins so that you can properly store them to preserve their beauty and also know how to identify coins that have questionable color.