fluorspar

I suggested fluorspar rather than NaF because a backyard full of fluorspar is a pretty rock garden, while a backyard full of NaF is a toxic waste dump.

I'd think neutralizing HF with chalk would yield fluorspar, which is resistant to weathering even over geological timescales.

Maybe dumping fluorspar in your yard will get you arrested.

Why wouldn't you want fluorspar raining on your head?

I have distilled HF out of powdered fluorspar and sulfuric acid in a home lab, from a lead tube I rolled out of sheet.

The internet seems to suggest lime (->fluorspar) or soda lye (->NaF, used for fluoridating water) to neutralize HF, but that's two other substances you wouldn't want raining on your head...

By the way, fluorides and fluoride minerals are virtually impossible to regulate trade in: they have a large variety of legitimate uses from toothpaste to aluminum production, and fluorspar is a fairly common mineral in the world.

The actual study, as opposed to the press release, includes some discussion of "fluorspar", which is something quite different (although it's still not a metal or a rare earth...).