Aladdin’s ring, that conjured the lesser genie.
Jem, of Jem and the Holograms, used her red star earrings to switch back and forth between her regular and her rock-star persona.
In the film version of Diamonds are Forever, Blofeld’s cat wears a diamond-studded collar.
The headpiece of the Staff of Ra is a key plot device in Raiders of the Lost Ark. While it was originally an ornament forming the head of a staff, when Indy tracked it down it was being worn by his former teenage girlfriend as a necklace.
The sacred ruby of Hinduism, the Syamantaka, protects against earthquake, floods, or famines, and grants a considerable amount of gold every day.
In the Spider-Man comics, a magic ruby transformed John Jameson (son of J. Jonah Jameson) into the Man-Wolf. He didn’t exactly wear it as jewelry, though. It grafted itself onto his throat. You might say that the jewel wore the man.
The mistical Tiger necklace worn by Marvel’s White Tiger.
Clarification required. @Czarcasm, do you want only items with magic powers, or will any famous fictional item do?
Personally, I think that only magic items would make a more interesting list.
Oh, that reminds me: in Logan’s Run, the populace had jewels embedded in the palm of their hands that showed when it was time for them to go to Carousel.
Reminder: Jewelry, not jewels.
Famous fictional jewelry.
The jewel on Orion the cat’s belt in Men In Black. OK, sorta, kinda.
You can geta replica on eBay, etc.
How about the Skull Ring of the The Ghost who Walks, aka The Phantom?
I have another thread about that ring, and it doesn’t come in size 14, and resizing stainless steel is next to impossible.
And the two rings of The Phantom have already been mentioned.
The Necklace of Harmonia. Initially a wedding gift to Harmonia and Cadmus, it later brought misfortune to their descendants.
Brisingamen, a necklace or torc worn by Freya.
In Tolkien’s Silmarillion, there is a necklace called Nauglamir. It is not magical, but its beauty sparks greed that wreaks havoc on numerous Elves, Dwarves, and Men.
The Thing Ring
Used by a young Ben Grimm before his space accident. O’ve not seen this show but I hear it’s awful.
No amount of bad alcohol will remove the phrase “Thing Ring! Do your thing!” from my brain.
The Lensmen from EE ‘Doc’ Smith wore their lenses, but a/ I don’t know if that counts as jewellery, and b/ I can’t find any illustrations by the classic pulp space artists that depict this, so here’s a comic book version.
The ring Angel gave Buffy
(Based on a real ring style.)
The Gem of Amara
https://buffy.fandom.com/wiki/Gem_of_Amara
(Probably not.)
All of the members of the Legion of Superheroes had (will have?) supertechnological rings that granted them the ability to fly (unless they already had that ability).