Au vs 24-karat gold

Au and 24-karat gold both are nouns.

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

As nouns, 24-karat gold is a hyponym of Au; that is, 24-karat gold is a word with a more specific, narrower meaning than Au:

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
24-karat gold: 100 per cent gold

Other hyponyms of Au include pure gold, gold dust, green gold, guinea gold.

Au (noun)	24-karat gold (noun)
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	100 per cent gold