24-karat gold vs Au

24-karat gold and Au both are nouns.

Word	Noun	Adjective	Verb	Adverb
24-karat gold	Yes	No	No	No
Au	Yes	No	No	No

As nouns, Au is a hypernym of 24-karat gold; that is, Au is a word with a broader meaning than 24-karat gold:

24-karat gold: 100 per cent gold
Au: a soft yellow malleable ductile (trivalent and univalent) metallic element; occurs mainly as nuggets in rocks and alluvial deposits; does not react with most chemicals but is attacked by chlorine and aqua regia

Other hypernyms of 24-karat gold include atomic number 79, gold.

24-karat gold (noun)	Au (noun)
100 per cent gold	a soft yellow malleable ductile (trivalent and univalent) metallic element; occurs mainly as nuggets in rocks and alluvial deposits; does not react with most chemicals but is attacked by chlorine and aqua regia