Rubidium was discovered in 1861 by Robert Bunsen and Gustav Kirchhoff, using the newly invented technique of flame spectroscopy, which revealed elements by the coloured lines they cast in a spectrum. They named it from the Latin rubidus, deep red, after the two dark red lines by which they had detected it. Rubidium is a soft, silvery alkali metal, so reactive that it ignites in air and reacts violently with water. Rubidium has few large-scale uses, serving mainly in specialised glasses, in atomic clocks and in research, but its readiness to shed an electron makes it a standard working material in laser cooling and other precision atomic physics.
Rubidium has a genuinely long-lived radioactive isotope, rubidium-87, which decays into strontium-87 at a rate slow enough to date some of the oldest rocks and meteorites on Earth, making rubidium-strontium dating an important tool in geology alongside potassium-argon and uranium-lead methods. Compact rubidium atomic clocks, less precise than the caesium standard but far smaller and cheaper, are now built into GPS satellites, telecommunications networks and other systems that need a stable, portable timekeeping reference, a quiet but essential modern application for an element with almost no visible consumer use.