Which metal is used in atomic clocks for precision?

In 1967, the unit of time, a second, underwent a re-definition. Instead of just being a tiny fraction of a year, it was now defined as the time it takes for radiation that is absorbed or emitted by a cesium atom to undergo a certain number of cycles.

Cesium clocks, do not use the earth's revolution, but instead use a measure of microwave radiations that excite electrons in a cesium atom and uses this frequency of this radiation to provide the unit of time.

Today, all Global Positioning Systems or GPS require the precision of cesium atomic clocks. They are also used in cellphones and other internet applications, especially useful in the new 5G technologies. The accuracy of cesium clocks has been improving over decades and nowadays, using lasers, the measurement of time has improved by an order of magnitude.

This metal is of critical importance to technology and is so rare on Earth, that its mining is still in a nascent stage and of extreme importance to our future.