Selenium is a nonmetal, chemically related to sulfur and tellurium, and rarely occurs in its elemental state in nature. Selenium is an essential micronutrient, although it is toxic in large doses. It is used as a catalyst in many chemical reactions, in the manufacture of glass and ceramics; and due to its photovoltaic and photoconductive properties, in photocopying, photocells, light meters, solar cells, and in the toning of photographic prints.