C is for Calcite. Calcite has some fun properties. It is double refractive, meaning, that a crystal that exhibits rhobohedral structure, when viewed over an image or writing, gives a double image. Another interesting property is that when two thin pieces of calcite from the same crystals are stacked 180* to each other, they make a set of concentric circles. This may sound very unimpressive but the Air Force used this property to manufacture aiming sites in bombers.
Other fun C minerals include chalcopyrite, chalcocite, corundum,