Hydroxyapatite consists of Ca2+ ions surrounded by both PO42- and OH- ions, and has the chemical formula Ca5(OH)(PO4)3. It is also known as tricalcium phosphate and calcium hydroxyapatite. The unit cell ( the smallest unit containing some form of symmetry) consists of two triangular prismatic subcells. These form a rhombic prism.

Some people have found its structure so
interesting that they have made origami models of it.