Selenium (Se) is a non-metal with a gray or black appearance, and it's an essential trace element for many organisms. Resembling sulfur both in its appearance and in its ability to form polymers, selenium has a range of allotropes. It is primarily used in the electronics industry for photovoltaic cells and as a photosensitive material in xerography. Selenium also finds applications in the glass industry to decolorize glass and make red and yellow pigments. Additionally, it's utilized in metallurgy to refine certain metals. In the realm of optics, selenium is known for its photoconductive properties. It should be handled with care, as excessive exposure can lead to selenium poisoning, which has symptoms similar to arsenic poisoning.