noble metal vs platinum

noble metal and platinum both are nouns.

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

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

noble metal: any metal that is resistant to corrosion or oxidation
platinum: a heavy precious metallic element; grey-white and resistant to corroding; occurs in some nickel and copper ores and is also found native in some deposits

Other hyponyms of noble metal include atomic number 79, Au, gold, atomic number 78, Pt, Ag, atomic number 47, silver.

noble metal (noun)	platinum (noun)
any metal that is resistant to corrosion or oxidation	a heavy precious metallic element; grey-white and resistant to corroding; occurs in some nickel and copper ores and is also found native in some deposits