Crystal

In chemistry and mineralogy, a crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions.

The word crystal originates from the Greek word "Krystallos" meaning clear ice, as it was thought to be an especially solid form of water. The word once referred particularly to quartz, or "rock crystal".

Most metals encountered in everyday life are polycrystals. Crystals are often symmetrically intergrown to form crystal twins.