In total there are seven crystal systems&colon; triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and cubic. In total there are six crystal families&colon; triclinic, monoclinic, orthorhombic, tetragonal, hexagonal, and cubic. Numerous examples are known with cubic, tetragonal, rhombohedral, and orthorhombic symmetries. Zirconia exists in the monoclinic, tetragonal or cubic crystal system depending on the temperature. The tetragonal and orthorhombic forms of PbO occur naturally as rare minerals. It exists in three phases&colon; monoclinic, tetragonal and cubic. The orthorhombic and tetragonal phases are most common non-cubic variants. The compound crystallizes as tetragonal birefringent crystals. The salt crystallizes as tetragonal crystals. The crystal structure of zircon is tetragonal crystal system. In some cases, the tetragonal phase can be metastable. At room temperature it forms paraelectric crystals with tetragonal symmetry. Bismuth subcarbonate has a structure with a tetragonal unit cell. It crystallizes in the tetragonal crystal system. This phase has tetragonal symmetry and is metallic.