Apatite is the name of a group of phosphate minerals with similar chemical compositions and physical properties. They are an important constituent of phosphorite, a rock mined for its phosphorus content and used to make fertilizers, acids, and chemicals. Apatite has a relatively consistent hardness and serves as the index mineral for a hardness of five in the Mohs Hardness Scale. Specimens with excellent clarity and color are sometimes cut as faceted gemstones. Those with good color and translucence are cut as cabochons.
Apatite forms under a wide variety of conditions and is found in igneous, metamorphic, and sedimentary rocks. The most important deposits of apatite are in sedimentary rocks formed in marine and lacustrine environments. There, phosphatic organic debris (such as bones, teeth, scales, and fecal material) had accumulated and was mineralized during diagenesis. Some of these deposits contain enough phosphorus that they can be mined and used to produce fertilizers and chemical products.
Apatite occasionally occurs as well-formed hexagonal crystals in hydrothermal veins and pegmatite pockets. These crystals often have a very high clarity and a vivid color and have been cut into gems for collectors. Mineral collectors also enjoy these well-formed apatite crystals, and the prices paid for them often exceeds their value as gem rough.
Transparent specimens of apatite with vivid green, blue, yellow, or pink color and excellent clarity are often cut into faceted gemstones. Some stones are heat treated to improve their color. As a gemstone, apatite is more popular with gem collectors than it is with jewelry buyers. The mineral has a Mohs hardness of 5, breaks with parting, and is very brittle. These characteristics make it too fragile for use in most types of jewelry.