Apatite is a group of phosphate minerals that are widely distributed in all classes of rock – igneous, sedimentary, and metamorphic. The most common use of apatite is in the manufacture of fertilizer, as it is a source of phosphorus. It is also used in the production of pure chemicals, ceramics, silk, textiles, insecticides, sugar refining, and the manufacture of explosives. Apatite is occasionally used as a gemstone, and green and blue varieties, in finely divided form, are used in some toothpaste and other dental products. Additionally, apatite is a biological mineral with important applications in the chemical and pharmaceutical industries. For example, a special form of apatite found in bone may find use for bone grafting materials and other orthopedic or dental applications. Apatite is also used for fission track dating, a technique that determines the age of a mineral sample by its uranium content. Finally, apatite occasionally contains significant amounts of rare-earth elements and can be used as an ore for those metals.