In materials science, a crystal is a solid substance in which the atoms, molecules, or ions are arranged in an orderly
repeating pattern extending in all three spatial dimensions.
The word crystal is a loan from the ancient Greek word κρύσταλλος (krustallos), which had the same meaning, but
according to the ancient understanding of crystal. At root it means anything congealed by freezing, such as ice. 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.
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