In sedimentary rocks, it occurs as an amorphous paste known as collophane, and when massive, it forms the bulk of phosphorite deposits.  
This mineral's high relief, low birefringence, and uniaxial negative character distinguish it from beryl or peridot in thin section, though it was named because its hexagonal habit and color were confused with those minerals. It is also distinguished by the presence of pleochroic haloes, which form from radiation damage from substituent uranium and thorium. Its structure consists of tetrahedra laterally bonded by calcium, with fluorine, chlorine or hydroxyl ions located in its interstices. In sedimentary rocks, it occurs as an amorphous paste known as collophane, and when massive, it forms the bulk of phosphorite deposits.  FTP, name this common phosphate mineral assigned a Mohs hardness of 5, the major component of tooth enamel.
This mineral's high relief, low birefringence, and uniaxial negative character distinguish it from beryl or peridot in thin section, though it was named because its hexagonal habit and color were confused with those minerals.
apatite