Meet the Rarest Natural Element on Earth. It's so rare, there's just 30 grams TOTAL in Earth's crust. Named after the Greek word for unstable (astatos), Astatine is a naturally occurring semi-metal that results from the decay of uranium and thorium. In its most stable form - astatine-210 - it's got a half-life of just 8.1 hours, which means even if you did happen to stumble on some of it, half of it would be gone by the end of a work day.