noble metal vs Au

noble metal and Au both are nouns.

Word	Noun	Adjective	Verb	Adverb
noble metal	Yes	No	No	No
Au	Yes	No	No	No

As nouns, Au is a hyponym of noble metal; that is, Au is a word with a more specific, narrower meaning than noble metal:

noble metal: any metal that is resistant to corrosion or oxidation
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 hyponyms of noble metal include atomic number 79, gold, atomic number 78, platinum, Pt, Ag, atomic number 47, silver.

noble metal (noun)	Au (noun)
any metal that is resistant to corrosion or oxidation	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