The toning of coins is a chemical reaction that usually occurs with certain types of paper, cardboard, and canvas. Often times you will come across a coin that is only toned on one side. This occurs because the coin was in a coin folder or at the end of a bank roll for a long time. A coin that is toned on both sides is synthetically toned 99.9% of the time. If you want to synthetically tone a coin you probably want to wrap the coin loosely in canvas or cheap napkin or paper towel (the brown kind). This reaction will take a long time, but it can be sped up by adding energy to the system, in this case: heat. The trick is to heat them slowly. The best way to do this is to place the wrapped coins near or on a heat vent or a water heater.
If you want to test this out just for fun, all you have to do is get a torch and a junk quarter. First heat the torch up so you have a tight crackling flame and then place the quarter at the peak of the cone in the flame (use tongs). You will observe the coin heating up really quickly. Right before the quarter gets white hot, the rainbow tone will shimmer from the center of the coin to the outside really quickly. When toning a coin, the goal is to stop at this stage of the reaction. Not an easy feat.