An element belonging to the alkaline earth metals. Symbol, Be. Beryl and bertrandite are the most important commercial sources of the element and its compounds. Aquamarine and emerald are the precious forms of beryl. Used in nonsparking tools, high-speed aircraft, missiles, spacecraft, communications satellites, and X-ray lithography for 297 microminiature integrated circuits, as well as in nuclear reactors and computers. Beryllium and its salts are toxic.