Set of six glas crystal models, late 19th century.
© Science Museum / Science & Society Picture Library
The set represents six crystal systems. All known crystalline substances crystallize according to one of seven systems, defined by planes of mirror symmetry and axes of rotational symmetry. Because crystal symmetry is related to internal molecular structure, any given mineral will always crystallize according to the same system, although this does not mean that the crystals will always grow to be exactly the same shape.