Which Natural Element Is the Rarest on Earth?

Astatine (At, element 85) is the rarest naturally occurring element on Earth. It is produced by the radioactive decay of heavier elements (uranium and thorium) but is itself highly unstable: its longest-lived isotope, astatine-210, has a half-life of just 8.1 hours. As a result, only about 25-30 grams of astatine exist in the entire Earth crust at any moment. Scientists have never seen a sample large enough to observe with the naked eye. Francium (element 87) is a close second in rarity. Both elements are known almost entirely from their radioactive properties and extremely small experimental samples produced in particle accelerators.