Selenium is a non-metallic element with unique properties that bridge the gap between metals and non-metals. The most common form is a gray, crystalline solid known for its distinct photoconductive and semiconductor properties. This means its electrical conductivity increases dramatically when exposed to light, making it invaluable in electronics. It was historically used in early rectifiers and photocells. Selenium is also used as a decolorizing agent in the glass industry to neutralize the greenish tint from iron impurities, and it can produce vibrant red glass. In metallurgy, it is added to stainless steel and copper alloys to improve their machinability, allowing for easier shaping and cutting. Additionally, it's used to create striking red and orange pigments for plastics, paints, and ceramics.