What is the rarest natural element?

The rarest naturally occurring element on Earth is astatine. There is less than one gram present in the Earth’s crust at any given time^[3][5]. Astatine is an extremely rare semi-metal that results from the decay of uranium and thorium and is highly unstable, with its most stable isotope, astatine-210, having a half-life of only 8.1 hours^[2][3]. The scarcity and rapid decay of astatine make it challenging to study, and any macroscopic specimen would immediately vaporize due to the heat of its own radioactivity^[3][5].