necklace vs jewellery

necklace and jewellery both are nouns.

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

As nouns, jewellery is a hypernym of necklace; that is, jewellery is a word with a broader meaning than necklace:

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

Other hypernyms of necklace include jewelry.

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