Cesium opened the age of spectroscopic discovery: Bunsen and Kirchhoff's new method let them see, in the coloured lines of a flame, the presence of an element too dilute to isolate by ordinary chemistry, and the sky-blue lines that named caesium were the first such revelation, followed within months by rubidium. The element later took on a deeper significance for measurement itself. Since 1967 the international definition of the second has been fixed by the frequency of a particular transition in the cesium-133 atom, so that the fundamental unit of time is now counted out by the vibrations of this blue-lined metal. Cesium has no ancient symbolism; its dignity is that of precision, naming the very beat of modern time.
Caesium also carries a harder modern reputation: the radioactive isotope caesium-137, a common fission product, was the dominant long-lived contaminant released at both the 1986 Chernobyl and 2011 Fukushima nuclear accidents, its roughly thirty-year half-life and chemical similarity to potassium letting it enter soil, crops and the food chain for decades afterward, making caesium monitoring a standard part of nuclear accident response and land remediation ever since. Most of the world's commercial caesium is refined from pollucite, a caesium aluminosilicate mineral, with the single largest known deposit at Bernic Lake in Manitoba, Canada, supplying much of the metal used in drilling fluids and atomic clocks alike.