Lutetium is a metal element with the chemical symbol Lu. The corresponding elemental form of lutetium is a silver-white metal, making it the hardest and densest metal among the rare earth elements. Its melting point is 1663°C, boiling point is 3395°C, and density is 9.8404. Lutetium is relatively stable in air; the oxide of lutetium is a colorless crystal that dissolves in acid to form colorless salts. Lutetium is mainly used for research purposes, with few other uses. It dissolves in dilute acid and reacts slowly with water. Its salt is colorless, and its oxide is white. The naturally occurring isotopes are: 175Lu with a half-life of 2.1×10^10 years and 176Lu, a beta emitter with a half-life of 2.1×10^10 years. Its natural abundance is very low, and it is quite expensive. It is produced by reducing calcium fluoride LuF₃·2H₂O, and is used in the atomic energy industry.