A specimen of purple fluorite (calcium fluoride). Fluorite is colourless when pure, but the presence of impurities gives a variety of colours including yellow, green, blue, pink, purple, and even black. It is found in many geological environments, from formations associated with metallic ores such as lead and silver, to sedimentary deposits associated with gypsum, dolomite and calcite. Fluorite is used in the production of hydrofluoric acid, which is important to the pottery, optical and plastics industries. Very clear crystals are used as apochromatic lenses and spectrographic prisms.