A divalent element, beryllium is found naturally only combined with other elements in minerals. Notable gemstones which contain beryllium include beryl (aquamarine, emerald) and chrysoberyl. The free element is a steel-gray, strong, lightweight brittle alkaline earth metal. It is primarily used as a hardening agent in alloys, notably beryllium copper. Structurally, beryllium's very low density (1.85 times that of water), high melting point (1287 °C), high temperature stability and low coefficient of thermal expansion, make it in many ways an ideal aerospace material, and it has been used in rocket nozzles and is a significant component of planned space telescopes. Because of its relatively high transparency to X-rays and other ionizing radiation types, beryllium also has a number of uses as filters and windows for radiation and particle physics experiments.
Commercial use of beryllium metal presents technical challenges due to the toxicity (especially by inhalation) of beryllium-containing dusts. Beryllium produces a direct corrosive effect to tissue, and can cause a chronic life-threatening allergic disease called berylliosis in susceptible persons. Because it is not synthesized in stars, beryllium is a relatively rare element in both the Earth and the universe. The element is not known to be necessary or useful for either plant or animal life.
