So, who was it that sent me the weed?
Cesium* is the most sublime and electropositive of all true elements. It was discovered spectroscopically, in mineral water from Durkheim Germany by Robert Bunsen (of burner fame) and Gustaff Kirchhoff in 1860. The basis of the discovery was two, beautiful sky-blue lines from which it takes its name (Latin, caesius, sky-blue). Atomic number 55, Cesium is the heaviest of the natural alkali metals.
Cesium occurs naturally in the minerals lepidolite and pollucite, the latter a hydrated aluminum cesium silicate. The richest known deposit occurs at Bernic Lake, Manitoba, where pollucite deposits averaging over 20 percent Cesium are estimated to exceed 300,000 metric tons. (Bernic Lake is also the site of the annual Cesium festival on February 24 where tons of Cesium are burned in the snow, and other Cesium madness prevails.)
Cesium is an alkali metal, in the same group as lithium, sodium, potassium, and rubidium, and is similarly reactive, but to a much higher degree due to its extreme electropositivity. It reacts explosively with water, and with ice down to -116 C. In air, it catches fire spontaneously and burns with a brilliant sky-blue flame. Its hydroxide is the most powerful aqueous base known, and will eat through glass, flesh, bone, and numerous other substances.
Natural Cesium consists of a single stable isotope, Cs 133. Thirty other radioactive isotopes are known, filling the range from Cs 114 to Cs 145. Cesium 137 (half-life 30.17y) is an important fission product, and one of the most biologically hazardous components of radioactive waste and nuclear fallout. Cesium 137 is also important as a source of high-energy X-rays in radiotherapy.
In addition to being a pyrotechnic IR emitter and radiotherapy agent, Cesium is used as a getter in vacuum systems due to its high affinity for oxygen. It has also been used experimentally in ion propulsion systems for spacecraft, again because of its low ionization potential. Probably the best-known use is in Cesium beam atomic clocks, which are among the most accurate time measuring devices in existence. They utilize a 9193 MHz hyperfine transition in the Cs 133 atom as the the fundamental oscillator. The best of these clocks have long-term stabilities of a few parts in 10^14, or about one second in a million years.
There is exactly one joke about Cesium. It goes like this. So these two Cesium atoms are walking down the street. Says the first Cesium atom, "Oh my God! I think I'm missing an electron." Says the second, "Are you sure?" Says the first, "I'm positive".
Shamelessly lifted and edited down from the alt.cesium FAQ
* The official IUPAC spelling and traditional British spelling of this element is "caesium." The United States still uses "cesium," a legitimate alternate spelling according to IUPAC (see also aluminum/aluminium). Until the alt.cesium newsgroup changes its name to conform to the new spelling, this writeup will remain stubbornly Americentric with the old one.