Double Refraction: -0.002 to -0.004
Apatite is generalized named for three different minerals with very similar properties and chemical structure, fluorapatite, chlorapatite and hydroxylapatite. The only major difference is the type of ions present in the first set of parenthesees in the chemical composition. Due to this lack of difference, these three minerals are almost impossible to tell apart on the site or in the mine. A crystal of apatite will consist of all three different minerals, although stones close to 100% of one type have been found in the wild.
Apatite's name comes from a Greek word meaning to deceive. Apatite can come in various solid colors such as yellow, green, blue, and violet. Some crystals of apatite are colorless as well. This causes a lot of confusion between apatite and other minerals, like olivine, peridot and beryl. Apatite is too soft to be confused with any precious gems like ruby, sapphire, topaz or diamond.
Apatite can be found in some hydrothermal vents and in iron-rich igenous rocks. It has also been found inside of matrices comprised of sedimentary and metamorphic rocks. Major apatite mines are located in Brazil, Burma, Sri Lanka, India, The Malagasay Republic, Mexico and the US state of Maine. Apatite is rarely used as a jewel, as it's so soft it can be scratched by a relatively sharp knife. Instead, it is often used as a source of phosphorus used in fertilizers.
Precious Stones, by Dr. Max Bauer. Charles E. Tuttle Company: Rutland Vermont and Tokyo, Japan, 1969
Gemstones of the World, by Walter Schumann. Sterling Publishing Co., New York, 1979
Simon and Schuster's guide to Rocks and Minerals, Simon and Schuster Inc. New York, 1978