Most mineral gemstones are crystalline, with their atoms arranged in regular and symmetrical patterns like a lattice. Crystalline minerals may consist of a single crystal,or of many in a group. Crystalline minerals are made up of a number of flat surfaces called faces. Crystals are classified into six different systems according to the “minimum symmetry” of their faces. This depends on a crystals “axes of symmetry” – imaginary lines (shown in the diagrams here) around which a crystal may rotate and still show identical aspects.
Crystals in the Cubic or Isometric system has the highest symmetry. The minimum symmmetry is four threefold axes.
Typical Tetragonal crystal shapes include four-sided prisms and pyramids. This system is defined by one fourfold axis.
Some consider there are seven crystal systems rather than six and separate Hexagonal and Trigonal. This is because these systems share the same axes of symmetry.
The Monoclinic system has a minimum symmetry of one twofold axis. More minerals crystalline in the Monoclinic system than in any other. The term ‘Monoclinic’ means ‘one incline’.
The minimum symmetry of the Orthorhombic system is three twofold axes. The name ‘Orthorhombic’ means ‘perpendicular parallelogram’ (try repeating that as fast as you can!).
Triclinic crystals have no axis of symmetry, so gemstones within this system are the least symmetrical.