Why does graphite conduct electricity while diamond does not? The difference in their appearances is one clue. The somewhat metallic luster of graphite announces its capacity to conduct electricity. Metals conduct electricity, while transparent substances, including diamond, are poor electrical conductors. However, rare diamonds, particularly the gray-to-blue ones called type IIB, are semiconductors, and are somewhat conductive. Both diamond and graphite have important applications in electronics: Diamond may be used as either an insulator (nonconductor) or a semiconductor, and graphite is commonly used as a conductor.

A substance is an electrical conductor if very little energy is required to move an electron from its bonded condition -- in the so-called valence band -- to a mobile condition in the conduction band. Such conductors are metallic looking because light is moving the electrons, too. In nonconductors (insulators) a large energy barrier exists between the valence band and the conduction band. This barrier must be "jumped" to allow conduction. Semiconductors such as type IIB diamonds have impurities that shrink the energy jump to the conduction band. These diamonds are moderate conductors of electricity.