Selenium

Selenium is a nonmetal related chemically to sulfur. It can form a red powder, a glassy black solid, or gray crystals with a metallic appearance. The gray form conducts electricity more readily when light falls on it, a property that has made selenium useful in light sensors. In nature, selenium usually replaces a small amount of sulfur in metal sulfide ores, especially copper ores. Industry recovers it mainly as a by-product from residues left during copper refining. Selenium compounds help glassmakers counteract unwanted tints caused by iron impurities or produce red glass. Our bodies also need tiny amounts of selenium, incorporated into proteins that help protect cells and support thyroid function, but too much is toxic.