Some facts about Beryllium:
Beryllium is the lightest of the alkaline metals (group 2) and has an atomic number of 4 and mass of 9.012 amu. Its elemental symbol is Be. It was discovered in 1797 and was first isolated in 1828.
Beryllium is very rare in the universe and on earth, and is usually found in compounds. Beryllium is found in beryl (beryllium aluminium cyclosilicate Be3Al2(SiO3)6 ) gemstones such as emeralds and aquamarines. While beryllium compounds have many uses such as radiation windows, alloys, mirrors, magnetics, electronics, and acoustics, it is toxic and can cause severe allergies or tissue damage. Beryllium is chemically similar to magnesium and can compete with magnesium to displace it as a cofactor, causing many enzymes (which are dependent on Mg2+ cofactors) in the body to malfunction.
More info can be found on the wikipedia article