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Crystal

Gemstones

Although the apparent perfection of gems is a major source of their attraction, the rich colors of many gemstones are due to tiny impurities of colored metal ions within the crystal structure. Table 2 lists some common gemstones and their crystalline structures.

The value and desirable properties of crystals promote scientific attempts to synthesize them. Although methods of synthesizing larger diamonds are expensive, diamond films can be made cheaply by a method called chemical vapor deposition (CVD). The technique involves methane and hydrogen gases, a surface on which the film can deposit, and a microwave oven. Energy from microwaves breaks the bonds in the gases, and, after a series of reactions, carbon films in the form of diamond are produced. The method holds much promise for: a) the tool and cutting industry (because diamond is the hardest known substance); b) electronics applications (because diamond is a conductor of heat, but not electricity); and c) medical applications (because it is tissue-compatible and tough, making it suitable for joint replacements, heart valves, etc.).

See also Diffraction.

Resources

Books

Hall, Judy. The Illustrated Guide To Crystals. London: Sterling Publications, 2000.

Hankin, Rosie. Rocks, Crystals & Minerals: Complete Identifier. New York: Knickerbocker Press, 1999.

Knight, Sirona. Pocket Guide to Crystals and Gemstones. Berkeley, CA: Crossing Press, 1998.

Lima-de-Faria, J., ed. Historical Atlas of Crystallography. Published for The International Union of Crystallography by Dordrecht: Boston: Kluwer Academic Publishers, 1990.


Massimo D. Bezoari

KEY TERMS

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Close-packing

—The positioning of atoms, ions, or molecules in a crystal in such a way that the amount of vacant space is minimal.

Covalent bond

—A chemical bond formed when two atoms share a pair of electrons with each other.

Diffraction

—A wave-like property of light: when a ray of light passes through a tiny opening it spreads out in all directions, as though the opening is the light source.

Electrostatic attraction

—The force of attraction between oppositely charged particles, as in ionic bonding.

Ionic compound

—A compound consisting of positive ions (usually, metal ions) and negative ions (nonmetal ions) held together by electrostatic attraction.

Lattice

—A pattern obtained by regular repetition of points in three dimensions.

Liquid crystal

—A compound consisting of particles which are highly ordered in some directions, but not in others.

Macromolecule

—A giant molecule consisting of repeating units of small molecules linked by covalent bonds.

Periodic Table

—A classification of the known elements, based upon their atomic numbers (the numbers of protons in the nuclei).

Unit cell

—The simplest three-dimensional repeating structure in a crystal lattice.

Additional topics

Science EncyclopediaScience & Philosophy: Cosine to Cyano groupCrystal - Common Classes Of Crystalline Solids, Internal Structures Of Metallic Crystals, Common Internal Structures Of Crystals Of Ionic Solids