We must start with the old and newer definitions of crystal. The old definition talked about a sold that has repeating units of molecules in a fixed order or pattern. However, this pattern is usually based on magnetic forces between the molecules. If the forces are very strong, the crystal is very solid and hard, like a diamond. 

This model of a crystal explains some of it’s behaviors. The first and most important being resonance. Because of it’s repeating pattern, other repeating energy patterns such as vibrations in air (sound) or the electro-magnetic spectrum (including light), can interact with the “spacing” of the crystal, causing “resonance”. Resonance occurs when two oscillating things coordinate or synchronize with each other. The vibrations of one can be transmitted and received by the other. 

Crystals are at the interface of matter and energy, and for this reason can help understand some of the mysteries of life. Crystals are created when molecules (matter) form a pattern (or order) that allows them to gain important functions. 

Information. 

The newer definition of crystal has been influenced by the discovery of the “Fourth Phase” of water, where this phase is in between

Liquid