 The symmetry of a crystal is constrained by the requirement that the unit cells stack perfectly with no gaps.There are 219 possible crystal symmetries, called crystallographic space groups.These are grouped into 7 crystal systems, such as cubic crystal system (where the crystals may form cubes or rectangular boxes, such as Halite (mineral) shown at right) or hexagonal crystal system (where the crystals may form hexagons, such as ordinary water ice).