The Physics of a Crystal Most simple chemical compounds consist of crystals. If you were to examine a lump of crystalline material very closely, you would be able to see tiny individual crystals. All crystals have a definite geometric shape, determined by the way the atoms are linked together. Mineralogists recognize 32 different classes of crystal, which are grouped into seven crystal systems. Crystal systems are described by their axis, which are imaginary lines that join the center of opposing faces of a crystal. For example, if a cubic crystal has three sets of opposing faces, it has three axes. They are all of equal length and are all at right angles to each other