Selenium

Selenium is a chemical element with symbol Se and atomic number 34. It rarely occurs in its elemental state in nature, or as pure ore compounds. Selenium is found in metal sulfide ores, where it partially replaces the sulfur.
Native (i.e., elemental) selenium is a rare mineral, which does not usually form good crystals, but, when it does, they are steep rhombohedra or tiny acicular (hair-like) crystals.
Selenium salts are toxic in large amounts, but trace amounts are necessary for cellular function in many organisms, including all animals.
Certain solids are selenium-rich, and selenium can be bioconcentrated by certain plants. In soils, selenium most often occurs in soluble forms such as selenate (analogous to sulfate), which are leached into rivers very easily by runoff.[36][37] Ocean water contains significant amounts of selenium.[40][41]
Anthropogenic sources of selenium include coal burning and the mining and smelting of sulfide ores. Although selenium is an essential trace element, it is toxic if taken in excess.
Signs and symptoms of selenosis include a garlic odor on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability, and neurological damage. Extreme cases of selenosis can result in cirrhosis of the liver, pulmonary edema, and death. Elemental selenium and most metallic selenides have relatively low toxicities because of their low bioavailability.
By contrast, selenates and selenites are very toxic, having an oxidant mode of action similar to that of arsenic trioxide. The chronic toxic dose of selenite for humans is about 2400 to 3000 micrograms of selenium per day for a long time.
Hydrogen selenide is an extremely toxic, corrosive gas. Selenium also occurs in organic compounds, such as dimethyl selenide, selenomethionine, selenocysteine and methylselenocysteine, all of which have high bioavailability and are toxic in large doses.