Crystals of minerals are classified according to their crystal system , so they can be monoclinic, triclinic , cubic, tetragonal, hexagonal, and orthorhombic. The crystal systems depend on the relation between the crystal faces and their angles. All minerals have crystals with their own unique crystal system. The crystal systems are forming due to multiple factors, the most important include the location of their formation (surface, below surface and how deep below surface), the pressure, the temperature, even the age, all of which determine what kind of crystal system will form and thus determine the type of mineral.