Light micrograph of cast iron using differential interference contrast (DIC) lighting. Steel & cast iron are made by adding small amounts of carbon to iron. When the carbon level is greater than 4%, discrete flakes or particles of carbon are formed & the steel is classed as cast iron. This specimen is a \spheroidal cast iron\" which, as a result of adding 0.04-0.06% magnesium, contains its excess carbon as graphite spheres (brown). The spheres make this cast iron relatively ductile. The smooth red-blue background is ferrite; the grainy areas are pearlite containing tiny spheres of cementite. Magnification: x100."