The Bajoran earring was an elaborate piece of jewelry traditionally worn on the right ear by the Bajoran people as a symbol of their faith. Each person's earring was individualized and bore the symbol of their family. Prior to the Cardassian Occupation of Bajor, these earrings also indicated a person's D'jarra (their social caste). (DS9: "Accession")
In 2346, a Bajoran earring made of ultritium resin was used in an assassination attempt on Gul Dukat, who was at that time the Prefect of Terok Nor. Undetectable by Cardassian sensors, the bomb had a three-minute timer which could not be disarmed. When detonated, the explosion had a radius of twenty meters. Dukat's quarters were destroyed and two Cardassian guards were killed, but Dukat and his comfort woman Kira Meru escaped in time. (DS9: "Wrongs Darker Than Death or Night")
Major Kira Nerys wore an earring, which was made for her by Lupaza using diamide-laced beritium salvaged from a Cardassian skimmer ambushed by the Shakaar resistance cell. (DS9: "The Search, Part I", "The Darkness and the Light")
The wearing of Bajoran earrings was generally against Starfleet uniform code. A special exception was made for Ensign Ro Laren by Captain Jean-Luc Picard when he recruited her to serve aboard the USS Enterprise-D in 2368. (TNG: "Ensign Ro") Tuvok ordered the Bajoran Gerron to remove his earring when the former Maquis member was being trained to participate as a Starfleet officer in 2371. (VOY: "Learning Curve") When Kira Nerys was given a temporary Starfleet commission of Commander in 2375 she continued to wear her earring while in Starfleet uniform. (DS9: "When It Rains...")
Quark wore an extra large Bajoran earring during the Bajoran Gratitude Festival when he was selling latinum plated scroll inscription pens. Some Ferengi waiters had oversized earrings for the occasion. (DS9: "Fascination") Rom too wore an extra large Bajoran earring, big enough to fit his Ferengi ear lobe, in the run-up to his marriage to Bajoran Dabo girl Leeta. (DS9: "Ferengi Love Songs")
Dukat wore an earring when he became leader of the Cult of the Pah-wraiths in 2375. His earring, along with the earrings of his followers, were red and worn on the left ear instead of the right, perhaps symbolizing the Pah-wraiths' opposition to the Prophets. (DS9: "Covenant")
When Kai Winn received her second vision from the Pah-wraiths, in which they informed her of their true nature, the Pah-wraith that had taken on Sisko's form handed her an earring of the Cult of the Pah-wraiths.