Astatineball

"There's no material safety data sheet for astatine. If there were, it would just be the word “NO” scrawled over and over in charred blood.."

- Randall Munroe, What if?

Astatine is a highly reactive element and is the rarest naturally occurring element in Earth's lithosphere. Since it is so radioactive, it decays away quickly and it is unknown what it would look like. A block of astatine would only last up to 8.1 hours. It would also burn down your neighbourhood/test facility. Of the first 101 elements, only Franciumball is less stable.