In another thread I mentioned my desire to acquire the ring of The Shadow, and it got me thinking about other famous/legendary fictional jewelry out there, such as the Flash’s ring (with his compressed outfit inside it) and the incredibly powerful Green Lantern ring. What other bits of fictional jewelry (rings, bracelets, necklaces etc.) comes to mind?
Three Rings for the Elven-kings under the sky,
Seven for the Dwarf-lords in halls of stone,
Nine for Mortal Men, doomed to die,
One for the Dark Lord on his dark throne
In the Land of Mordor where the Shadows lie.
One Ring to rule them all, One Ring to find them,
One Ring to bring them all and in the darkness bind them.
In the Land of Mordor where the Shadows lie.
Fafnir’s Ring recovered by Sigurd.
Ring of Gyges which granted invisibility.
A ring for every finger and toe, and one for either an ear or a nose. Thank you, Tolkien.
I had to look up the name, but Odin’s Golden Ring, Draupnir which creates 8 new ring of the same weight every night.
The rings of the Schwartz, from Spaceballs.
The Elessar worn by King Aragorn. The stone is a green gem set in a silver eagle brooch.
There is a whole large list of Magic Rings and other jewelry from the D&D books. Probably best not to list those.
ETA:
Thomas Covenant’s wedding ring.of White Gold was magic in the world he Isekai.
Wonder Woman’s bracelets, and the Nega-Bands worn by Marvel’s Captain Marvel for awhile.
Does the Infinity Gauntlet count? Maybe only once the stones are added?
It would still be a glove, I think. Since neither the gems nor the gauntlet are actually worn as jewelry (as far as I know), I don’t think they become one if combined.
added: Oh my! It seems that Dr. Strange’s Eye Of Agamotto necklace contained the Time Stone Infinity Gem.
The Phantom had two rings: a skull ring for making impressions on the faces of bad guys and on his other hand, the design was crossed Ps, making an 'X." This was for making impressions in the hands of good guys, indicating they were under The Phantom’s protection.
The Mandarin in Marvel comic books has many rings, each with a deadly power.
Sinestro’s yellow ring is a bit different than Green Lanterns.
The necklace in Titanic:
This necklace isn’t quite iconic, but this scene is:
Tolkein also gave us the Silmarils, three powerful jewels that could be worn (and sometimes were).
The Eye of Ra medallion in Stargate.
In the story of Aladdin, there was a ring that also controlled a genie. The Genie of the Lamp was more powerful than the Genie of the Ring.
Green Lantern was originally a modern version of Aladdin.
Also, the headpiece of the Staff of Ra in Raiders of the Lost Ark.
Johnny Cash, Ring of Fire…
Open the window.
Do you hear that loud “NOOOOOOOO!” that seems to emanate from everywhere at once? 
Marvolo Gaunt’s ring and Salazar Slytherin’s locket.
Originally worn as a necklace, wasn’t it?
In the Dungeon Crawler Carl novels, Princess Donut – a Persian cat who becomes intelligent, as well as becoming a powerful spellcaster – finds, and wears, a powerful magical tiara, the Enchanted Crown of the Sepsis Whore.