Rubidium is a soft, silvery-white metal. It is slightly harder than potassium and softer than caesium. The metal is very reactive and tarnishes rapidly in air, forming a black oxide layer. Rubidium can be cut with a knife and has a melting point of 39.31 °C (102.76 °F) and a boiling point of 688 °C (1,270 °F).