Coin toning occurs when a coin’s metal reacts with environmental elements such as oxygen, sulfur compounds, humidity, light, and even oils from handling, forming a thin layer of oxidation or tarnish on the surface . This process can take years or decades, gradually producing colors ranging from subtle golds and browns to vibrant blues, purples, and rainbow patterns . Toning is most common on silver coins but can also appear on gold, copper, and cupronickel coins