Fluorine

Fluorine is reasonably abundant but not too abundant chemical element.
It is by no means as abundant as it's heavier same-group-sibling-element chlorine.
Chlorine can be grabbed from seawater in nigh boundless amounts.

Fluorine:

• Can be used for nanoscale surface passivation very similar to hydrogen
• Likes to snatch lots of electrons from sulfur (SF₆ is a highly stable molecular compound)

Fluorine likes to form

• highly stable compounds.
  
• quite dangerous dangerous compounds (insidiously acidic and toxic)

In case of extreme treatment even the stable ones decompose and become dangerous.
Thus one might want to:

• use fluorine in moderation
• not use fluorine in combustible products or potentially via heat exposure out-gassing products. (Note: Most gem-gum products not made solely from air will likely be incombustible.)
  

This is basically for the same reasons we want to avoid the usage of PVC plastic (polyvinyl chloride)
Just that fluorine is even more aggressive when released by combustion.

Related

• Chemical element
• Periodic table of elements
• Limits of construction kit analogy
• Oddball compound#Rather inert compounds with fluorine