If I recall correctly, Thallium is essential to the growth of Tree-of-Life.
Thallium and Thulium are two different elements.
I came in to say his one. While natural (and thus acceptable to the OP) it is extremely unstable, so supposedly there are only around 30g of it in the Earth at any one time.
But in researching it I find that Astatine is even rarer, although only barely - there is only about 28g of it at any moment.
I’ll second Thulium. Naturally occurring and stable, though incredibly rare. No known biological function. Very limited industrial use.
Beryllium - popular use in non-sparking tools.
My vote is for Osmium – Only known use is in the tagline, “Well that joke went over like an Osmium balloon.”
Electron microscropy.
In addition to being hard to find, “Unobtainium” is largely useless.
Does radon (heavy radioactive gas) have any use?
Well, there’s the whole household radon detection and remediation industry that would collapse without the existence of radon.
According to Wikipedia radon can be used to detect faults. Elevating levels from a norm may also be an indicator for geological activity.
Historically it was used to poison gullible people with radon and radium treatments for health.
A genuine beneficial use had been in the use of gold-plated seeds for Brachytherapy - but it seems that other elements and isotopes are better favored, now, for that treatment.