The Chemistry of the Change in Your Pocket
If you look through the change in your pocket, and the copper coins in particular you will notice that they appear to be varying shades of red, orange or brown. Some are bright and shiny orange colour, others are a much duller shade of brown. Historically original coinage was often made of gold, partly for this very reason. Gold is a noble metal and the gold Doubloons, Florins, Ducats and Krugerrands were (and still are) precious partly because they are made of a metal which doesn't oxidise or tarnish.