a radioactive chemical element with the chemical symbol At and atomic number 85. It occurs on Earth only as the result of the radioactive decay of certain heavier elements. All of its isotopes are short-lived; the most stable is astatine-210, with a half-life of 8.1 hours. Accordingly, much less is known about astatine than most other elements. The observed properties are consistent with it being a heavier analog of iodine; many other properties have been estimated based on this resemblance.

Astatine occurs in vanishingly small quantities in the natural decay chains of uranium and thorium minerals. You can't see any of it in this rock, but a few atoms are (probably) there from time to time.