Something to know is that nickels are composed of Cupronickel Planchets. That's a mixture of 75% Copper and 25% Nickel Same thing goes for Dimes, Quarters and Half Dollars which are composed of Cupronickel clad with a copper core. So the Cupronickel clad and Cupronickel nickels will tone when exposed for an extended period of time. Cupronickel coins buried in dirt, soil and sand will be affected. Harsh chemicals such as soda, brackish water will also affect the coins. Here are many examples of my metal detected coins with environmental exposure toning damage.