Lutetium was the last natural rare earth element to be discovered. It is a silvery-white metal that is corrosion-resistant in dry air. Lutetium metal is slightly unstable in the air at standard conditions, but it easily burns at 150 °C to form lutetium oxide. Lutetium Metal (Lu Metal) is the densest and the highest-melting rare-earth element and the last member of the lanthanide series.