Simple gold band wedding ring belonging to Jin, with the inscription in Korean on the inside of the band that reads, "We will never be apart."  When Locke decides to bring the Oceanic Six back to the Island, Jin begins to have misgivings about letting Sun return to the place of "death."  As a result, he gives Locke his wedding ring to give to Sun.  This would tell Sun that he had died and would take away her own reason to return.  For his part, Locke keeps his promise.  However, after Ben strangles Locke and takes the ring, Ben uses it to his advantage and convinces Sun that Jin is indeed alive and that she should return to the Island. 

(Season 5, Episode 5, "This Place is Death")