Selenium has several allotropic forms, but only three are generally recognized. Amorphous selenium is either red, in powder form, or black, in vitreous, or glassy, form. The most stable form of the element, crystalline hexagonal selenium, is a metallic gray, while crystalline monoclinic selenium is a deep red. The element resembles sulfur in its makeup.
Selenium, an essential trace element involved in defense against oxidative stress, may prevent cancer and cardiovascular disease. Selenium is a nutritionally essential element. People need selenium for healthy joints, heart and eyes. It plays a critical role in DNA synthesis, the immune system and the reproductive system. It also helps fight cancer and other diseases.