Point defects in crystals consist of holes, interstitial atoms, and impurities. These point defects are important for physical and chemical properties of the crystals. In particular, impurities and electrons trapped in holes are responsible for optical properties of the crystals. In jewelry such as ruby and sapphire, d-d transition of impurity transition-metal atoms is the origin of its sparkling color. Similarly to the colors of ruby and sapphire, a crystal such as NaCl and MgO can be colored. The color of such crystals is due to electronic transitions of color centers, which consist of electrons trapped in hole sites. Interactions between photons and electrons in the color centers are explained from the basic theory of the quantum mechanics. The electronic structures of the F and U centers in NaCl and of various F centers in MgO are discussed from results obtained by recent molecular orbital calculations.
