Grey (crystalline) and black, red (amorphous) forms of Selenium
Selenium can be present as a red or black amorphous, or a red or grey crystalline structure, which is its most stable structure. Selenium has properties very similar to those of sulfur, however it is more metallic even though it is still classified as a non–metal. It is better termed as a metalliod. It acts as a semiconductor and therefore is often used in the manufacture of rectifiers, which are devices that convert alternating currents to direct currents. Selenium also has photo conductivity, which means that in the presence of light the electrical conductivity of selenium increases. It is also used in the drums of laser printers and copiers. In addition, it has found increased use now that lead has been removed from plumbing brasses.
It is rare to find selenium in its elemental form in nature, and so typically it must be removed through a refining process, usually involving copper. It can often be found in soils and in plant tissues that have bioaccumulated the element. In large doses, the element is toxic, however many animals require it as an essential micronutrient. For humans, it is an essential cofactor in maintaining the function of one's thyroid gland. In addition, some research has shown there to be a correlation between selenium deficient soils and an increased risk of contracting the HIV/AIDS virus.