The hexagonal bifrustum or truncated bipyramid is the fourth in an infinite series of bifrustum polyhedra. It has 12 trapezoid and 2 hexagonal faces. This polyhedron can be constructed by taking a hexagonal dipyramid and truncating the polar axis vertices, making it into two end-to-end frustums. A sapphire or ruby crystal is an example of a hexagonal bifrustum. It has been used for a decorative polyhedral monthly calendar, with one month on each trapezoid face. (regular and rhombic dodecahedra have also been used for calendar months.)