Which element is the rarest on Earth?

Astatine is a highly radioactive element that is so unstable that it exists only fleetingly in nature, formed as heavier elements decay. Scientists estimate that at any given moment the entire Earth crust contains less than one gram of astatine in total. Its isotopes decay so quickly that it is nearly impossible to gather a visible amount, and much of what is studied is made artificially in laboratories. Francium is another extremely rare and unstable element, with only tiny traces present at any time. These vanishingly rare elements sit at the frontier of what chemists can study.