A nonmetallic element, red in powder form, black in vitreous form, and metallic gray in stable crystalline form, resembling sulfur and found as an impurity in pyrites or obtained as a byproduct of electrolytic copper refining. It is widely used in rectifiers, as a semiconductor, and in xerography. Its photovoltaic and photoconductive actions make it useful in photocells, photographic exposure meters, and solar cells. Atomic number 34; atomic weight 78.96; melting point (gray) 221°C; boiling point (gray) 685°C; specific gravity (gray) 4.79; (vitreous) 4.28; valence 2, 4, or 6. See Periodic Table.