Crystal system - The primary method of classification of crystals. The Crystal system classifies crystals in six groups. They are: Isometric, Tetragonal, Hexagonal (which includes Trigonal), Orthorhombic, Monoclinic, and Triclinic. The crystal class, which classifies crystals into 32 crystal types, is a more precise classification of crystal groupings. (See Crystals in Mineral Properties for more information.)