jewellery vs necklace

jewellery and necklace both are nouns.

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

As nouns, necklace is a hyponym of jewellery; that is, necklace 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)
necklace: jewelry consisting of a cord or chain (often bearing gems) worn about the neck as an ornament (especially by women)

jewellery (noun)	necklace (noun)
an adornment (as a bracelet or ring or necklace) made of precious metals and set with gems (or imitation gems)	jewelry consisting of a cord or chain (often bearing gems) worn about the neck as an ornament (especially by women)