jewellery vs cufflink

jewellery and cufflink both are nouns.

Word	Noun	Adjective	Verb	Adverb
jewellery	Yes	No	No	No
cufflink	Yes	No	No	No

As nouns, cufflink is a hyponym of jewellery; that is, cufflink is a word with a more specific, narrower meaning than jewellery:

jewellery: an adornment (as a bracelet or ring or necklace) made of precious metals and set with gems (or imitation gems)
cufflink: jewelry consisting of one of a pair of linked buttons used to fasten the cuffs of a shirt

jewellery (noun)	cufflink (noun)
an adornment (as a bracelet or ring or necklace) made of precious metals and set with gems (or imitation gems)	jewelry consisting of one of a pair of linked buttons used to fasten the cuffs of a shirt