Hydroxyapatite

Hydroxyapatite consists of Ca²⁺ ions surrounded by both PO₄^2- 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.