The rarest naturally occurring element on Earth is astatine. Astatine is a member of the halogen group in the periodic table and is extremely rare due to its high radioactivity and short half-life, typically existing in trace amounts within uranium and thorium ores. It is estimated that there are only about 25 grams of astatine present in the Earth’s crust at any given time. Because of its rarity and radioactivity, it has limited practical applications. Astatine's properties are still studied in scientific research, particularly concerning its potential uses in medical treatments for cancer.