Color of Minerals
The color of a mineral’s surface is helpful in identifying the mineral. However, many minerals have variable colors. Calcite and quartz are examples of such minerals. Gold and silver are examples of minerals whose colors are constant and distinctive.
Chemical impurities in minerals generally cause color variations. Iron is a common impurity in minerals. The yellowish-brown or reddish-brown sphalerite of western Ohio contains very little iron. The black sphalerite of eastern Ohio, however, contains significant amounts of iron. When examining a mineral specimen for its color, you always should look at a clean, unweathered surface. This is because tarnishing sometimes changes the color of a mineral. For example, marcasite can tarnish to a dark brassy color or brown, quite unlike the color of a freshly collected specimen.