A Bajoran earring.
The earring of the Cult of the Pah-wraiths.
The Bajoran earring is an elaborate piece of jewelry traditionally worn on the right ear as a symbol of their faith. These earrings can be made from various metals, such as diamide-laced beritium. Each person's earring is individualized and bears the symbol of their family. Prior to the Occupation, these earrings also indicated one's D'jarra, or social caste. (DS9: "Accession")
Bajoran earrings are generally against Starfleet's uniform code. A special exception was made for Ensign Ro when Captain Picard recruited her. (TNG: "Ensign Ro")
Kira Nerys wore an earring. (Star Trek: Deep Space Nine)
Among the items stolen by a Dopterian in Quark's bar in 2369 was a Bajoran earring. (DS9: "The Forsaken")
Tuvok ordered the Bajoran Gerron to remove his earring when trying to train the former Maquis to participate as a Starfleet officer. (VOY: "Learning Curve")
Dukat wore an earring when he became leader of the Cult of the Pah-wraiths. (DS9: "Covenant")