Crystal, solid substance in which the individual molecules, atoms, or ions are arranged in a geometrical form. Almost all pure substances (chemical elements, compounds, mixtures) can form solid crystals. Substances that do not crystallize are called amorphous. Crystals may be formed from solutions, as salt crystals form when a pool of sea water evaporates in the sun; when liquids solidify, as when water freezes to ice or when molten rocks have solidified and have formed crystals; or when vapors solidify (iodine crystals can easily be made by heating some solid iodine in a closed container). Crystals form into definite shapes because the atoms in the substance always arrange themselves in a specific array called a lattice. The lattice consists of rows of atoms at various angles. The shapes of the crystals depend on the angles of the rows. The kind of lattice a crystal possesses can be found by passing X rays through the crystal. The lattice affects the X rays so that they emerge from the crystal with a pattern that can be recorded on photographic film, indicating the kind of crystal being studied. The study of crystals is called crystallography. Since crystals often form with corners missing from the basic shape, crystal systems are best described in terms of their axes, rather than their faces. The axes are imaginary lines across the crystals that join opposite faces. Cubic crystals have 3 equal axes at right angles. Salt and alum are cubic crystals, but whereas a salt crystal looks like a cube, an alum crystal has corners missing and is an 8-sided pyramid, or octahedron. Cubic crystals are also called isometric crystals. Tetragonal crystals also have 3 axes at right angles, but 1 axis is longer than the other 2, so the side faces are rectangular. Tin forms tetragonal crystals. In orthorhombic crystals, the 3 axes are at right angles but of different lengths. Sulfur forms orthorhombic crystals. Monoclinic crystals have 3 unequal axes, only 2 of which meet at right angles. Gypsum forms monoclinic crystals. Triclinic crystals have 3 unequal axes that do not meet at right angles. Copper sulfate forms triclinic crystals. Hexagonal crystals have 4 axes, 3 in one plane, equal in length and at 60° to each other, the fourth longer or shorter and at right angles to the others. Beryl forms hexagonal crystals. Trigonal crystals also have 4 axes, but only 3 basic side faces instead of 6. Quartz is a trigonal crystal. Several electrical effects occur in crystals. Piezoelectric crystals produce an electric signal when they are twisted out of shape and return to their original shape. Rochelle salt crystals can produce electric signals with the frequency range of sound waves and so are used in phonograph pickups, microphones, and hearing aids. Quartz crystals produce signals at only one frequency and so can be used in oscillators, electronic filters, and radio tuners. The electronic properties of a transistor depend on the arrangement of atoms in the lattice of the crystal. The crystals have to be very pure and are made with extreme care by a crystallization process called zone refining. Crystals also have interesting optical properties. Certain types can be used, for example, to produce polarized light.