Crystal systems 
The crystals are classified in seven systems according to the symmetry of their faces, defined by axes of symmetry around whose the crystal can turn while always presenting the same aspect. If this aspect repeats 2, 3, 4 or 6 times during a complete rotation (360°), this axis is binary, ternary, quaternary or senary. These seven systems are as follows :

The crystals of this system are strongly symmetrical: at least three quaternary axes.
Example of stone: Diamond 

The crystals of this system have a senary axis of symmetry.
Example of stone: Emerald 

The crystals of this system have a ternary axis of symmetry.
Example of stone: Ruby 

The crystals of this system have three binary axes of symmetry.
Example of stone: Topaz 

The crystals of this system have a quaternary axis of symmetry.
Example of stone: Zircon 

The crystals of this system have an in theory binary axial axis of symmetry.
Example of stone: Moon stone 

The crystals of this system do not have an axis of symmetry!
Example of stone: Turquoise 
