Francium – $5 Trillion/Kg

Francium is a highly radioactive substance, which is what makes it both extraordinarily harmful and equally invaluable. Being the second rarest substance in the Earth’s crust after Astatine, it is easily one of the most expensive elements in the world.

The life span of the material is just twenty-two minutes. Francium is used mainly in research labs due to the difficulty in attaining it, and its commercial uses are next to nil. Yet, this too is associated with the diagnostics for curing cancers and is not known to have an impact on the environment.