Astatine is the rarest naturally-occurring chemical element. Its symbol is At and it has the atomic number 85.
It's highly radioactive. Because of its short half-life (even the most stable isotope only has a half-life of around 8 hrs) it has been very difficult to study.
Although it decays very quickly, it's continuously being produced - in minute quantities - by radioactive processes in the decay of uranium ores. It's estimated that less than 1 gram is present in the Earth's crust at any given time.
It can be synthesized in the laboratory in small quantities, but it decays rapidly, and vaporizes so quickly due to the heat caused by its own radioactivity, that Its 'bulk properties' are still generally unknown. It's assumed to be somewhat similar to iodine, but possibly more metallic.
At and compounds derived from the element are under investigation for possible use in radioactive cancer therapy.