?l

Naw. Let's make an iron standard.

oxygen standard

Nitrogen standard

Feminist Standard

If we're going for scarcity perhaps consider a Francium or Astatine standard. Neither of those occurs in quantities of more than a few atoms at a time.

Ok, too scarce... In that case... Gold, Silver, Platinum, Palladium, Ruthenium, Rhodium, Iridium...