Cesium (“caesium,” outside the US) is not just a ringtone. Nor is it simply the atom used in ultra-precise clocks that make everything from the Internet to GPS possible, thanks to accurate time stamps.
Yes, cesium defines time itself.
Although it can be hazardous in some forms, cesium also is extremely useful in a variety of niche applications ranging from things like catalyzing chemical processes to cleaning sulfur out of crude oil to making fiber optic and other specialty glasses to building cellphone motion sensor devices.
In its manmade radioactive form, Cs137 is used for treating cancer and sterilizing equipment in modern medicine; and as cesium formate, it is rented out as a reusable brine to companies drilling oil and gas wells in high-temperature, high-pressure formations.
The United States depends on imports, mainly from a mine in Canada. And that mine closed in 2015, although its cesium stock is still sufficient to meet US and worldwide demand for the near future.