Tungsten is a rare metal found naturally on Earth almost exclusively combined with other elements in chemical compounds rather than alone. Its important ores include wolframite and scheelite. The free element is remarkable for its robustness, especially the fact that it has the highest melting point of all the elements discovered and also has the highest boiling point. Polycrystalline tungsten is an intrinsically brittle and hard material (under standard conditions, when uncombined), making it difficult to work. However, pure single-crystalline tungsten is more ductile and can be cut with a hard-steel hacksaw.
Tungsten’s many alloys have numerous applications, including incandescent light bulb filaments, X-ray tubes (as both the filament and target), electrodes in gas tungsten arc welding, super-alloys, and radiation shielding. Tungsten’s hardness and high density give it military applications in penetrating projectiles. Tungsten compounds are also often used as industrial catalysts.
Tungsten is the only metal from the third transition series that is known to occur in biomolecules, being found in a few species of bacteria and archaea. It is the heaviest element known to be essential to any living organism.