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A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered array extending in all three spatial dimensions. Various shapes of crystals are found in nature, and are dependent on the types of molecular bonds between the atoms to determine the structure. Snowflakes, diamonds, and common salt are examples of crystals.

Salt is an example of an ionic crystal. It has ionic bonds between the adjacent atoms in the fundamental salt crystal pattern or unit cell.

Solids without such ordering are known as amorphous solids. While quartz is crystalline silicon dioxide (SiO2), fused quartz has the same chemical composition but is an amorphous solid.

Most crystalline materials have a variety of crystallographic defects. The types and structures of these defects often control the properties of the materials.

Some crystalline materials may exhibit special electrical properties such as the ferroelectric effect. The behaviour of light in crystals is described by crystal optics.

See also: Crystal structure, crystal habit

In a household context, crystal (short for lead crystal) refers to silicate glasses made with lead and alkali oxides. Despite the confusions with its name, lead crystal or any form of glass is an amorphous solid.

Liquid crystals occupy a conceptual middle ground between the crystalline and the amorphous--they exhibit order in some dimensions, but amorphous-like disorder in others.

Crystal is also a slang term for methamphetamine.

There was a 1960s American girl group called The Crystals, best known for songs like "He's a Rebel" and "Then He Kissed Me".

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