I'm going to make a bold claim: no datatype has a quality natural unique key.

I am struggling to think of a counterexample. Maybe chemical elements? Where the natural key is of course the atomic number, not the chemical symbol...

Before you suggest 'zipcodes' or 'states', consider that zipcodes are not in any sense natural, and anyway, like state codes, are so US-centric that they don't belong as top level elements in most real database schemas.