Crystal, a solid body formed by atoms, ions, or molecules arranged in a regular, three-dimensional pattern. Gemstones, salt, snow, and metals are familiar examples of materials made of crystals. Some crystalline materials, such as gemstones, can be single crystals; others, such as metals, are made up of large numbers of small crystals, called grains, that are joined together.

The particles that form a crystal—that is, the atoms, ions, or molecules—arrange themselves into patterns because they attract one another in some directions more than in others. This pattern tends to give single crystals a characteristic external shape with symmetrically arranged flat surfaces. The smallest complete grouping of the particles that make up a crystal is called a unit cell; unit cells, in turn, make up the three-dimensional framework, or lattice, of the crystal.

Quasicrystals, like crystals, are solids formed by particles in an ordered pattern, but they have more than one kind of unit cell and the unit cells are repeated in a pattern that is not entirely regular. Any solid that is not a crystal or quasicrystal is called amorphous (without form). The particles that make up an amorphous solid attract one another equally in all directions, and no definite pattern results. Liquid crystals have properties of both liquids and crystals.

Crystals form when certain liquids or gases are cooled until they solidify. As the substance begins to solidify, some of the particles join into unit cells, usually on the surface of another solid. Other unit cells tend to form around existing ones, so that as the substance solidifies the crystals that first form tend to grow. A typical example of crystal growth is the formation of rock candy (lumps of crystallized sugar) around a string suspended in a cooling solution of sugar and water.

The form of a crystal depends upon the substance of which it is made. Under the same conditions, a particular substance will always crystallize in the same form. However, under different conditions of temperature and pressure, the same type of substance may crystallize in different forms. For example, carbon has two distinct crystalline forms—diamonds and graphite.

Crystallography is the study of the structure and growth of crystals, and their reactions to electrical and mechanical forces. X-ray pictures have revealed seven fundamental crystal forms, or systems.