The user has access to a piece of jewelry (rings, pendants, badges, etc.) that grants them various supernatural powers, some which amplifies one's powers, some which can give superpowers, some can even give binding curses which grants them unearthly powers.
Adrien using the Cat Miraculous (Miraculous Ladybug) to become Cat Noir.
Marinette using the Ladybug Miraculous (Miraculous Ladybug) to become Ladybug.
With the power of the Butterfly/Moth Miraculous, Hawk Moth (Miraculous Ladybug) has the power to turn people into supervillains.
Using his brooch as a vessel for an akuma, Hawk Moth was able to turn Kim Chiến Lê (Miraculous Ladybug) into Dark Cupid.
Using her wrist-mirror as a vessel for an akuma, Hawk Moth was able to turn Juleka Couffaine (Miraculous Ladybug)...
The Witches of Oz (Once Upon a Time) are all embued with great magical power...
...granted to them by the Oz Pendants.
The one Ring (Lord of the Rings) is an incredibly powerful artifact that grants invisibility to those who where it. Because of its connection to Sauron however, those who bear the ring are driven mad by it.
As a bearer of the One Ring, Gollum (Lord of the Rings) was able prolong his life using it, driven mad by its influence.
Jewelry-Based Horcruxes (Harry Potter) can empower those who wear them. Because of their evil nature, only those who possess evil natures themselves can take advantage of this.
Because of her evil nature, Dolores Umbridge (Harry Potter) was given an increase in magical power by wearing Salazar's locket, one of Voldemort's horcruxes.
The Eye of Odin (Gargoyles) is a magical jewel that grants the wearer great power, but it also corrupts them in a way depending on the wearer
With the Eye of Odin, Fox (Gargoyles) was transformed into a lycanthrope-like creature.
Goliath (Gargoyles) enhanced by the Eye of Odin.
Jewel Girl (Valkyrie Crusade) can gain a specific power from each jewel (Black Pearl:Exorcism.Hematite:Victory Inducement.etc)
ÄRMs (Marchen Awakens Romance) are magically enhanced jewelry that grant special powers to those who synchronize with them.