A crystal structure is a regular, repeating, three dimensional arrangement of bonded atoms. Though the generic term crystal is used to describe a single, solid crystal with defined crystal faces, much gem material is still crystalline even if its outward appearance is rough and irregular. This is because the internal structure within the material is still crystalline. It is the orderly repeating array of bonds with in crystalline material that gives rise to the ways in which it interacts with light, physical forces and chemical activity. It is these interactions that allow a gemmologist to do their job. For example, light traveling through a piece of blue glass will generally exit through the other side without much change other than showing blue. A similar looking aquamarine however, can split the light moving through its crystal structure into wavelengths vibrating in perpendicular directions (this will be explained in more detail later). This effect can be easily detected and used to differentiate aquamarine and glass imitation. Two important elements of describing a crystal are: Axis of Symmetry- an imaginary line which indicates a direction through a crystal structure about which the structure can be rotated to appear identical two, three, four or six time during one complete rotation. Plane of Symmetry- an imaginary mirror plane dividing a crystal structure into two parts. Either part can be reflected through the mirror and still appear identical to the other part. Crystal symmetry can be quite a taxing subject, so we will cover it on so we will just keep it at these basics. In part two we will move on to the crystal systems we classify gems into and look at the shapes and forms of common gem material.