A member of the phosphate group of minerals, the world's major source of phosphorus, found as variously coloured, glassy crystals, masses, or nodules.
Much of it has a chemical composition approximating Ca 5 (PO 4 ) 3 (F,Cl,OH). If not for its softness, apatite would be a popular gemstone; some of the material found is clear, but it is fragile and difficult to cut and polish.