What is the rarest element?

Astatine:

1. The chemical element astatine has the atomic number $85$ and the symbol $\mathrm{At}$.
2. It belongs to group $17$ and period $6$ of the periodic table.
3. It is the rarest naturally occurring element in the crust of the Earth and only appears as a decay product of several other heavier elements.
4. It is a naturally occurring semi-metal that results from the decay of uranium and thorium and is named from the Greek word for instability (astatos).
5. It has high boiling and melting point.
6. All of the astatine's isotopes have short half-lives, but only astatine$-210$ has the longest half-life at $8.1$ hours.

Therefore, Astatine is the rarest element.