The emerald lantern is a lightsource which is used during the Lunar Diplomacy quest. It is created by lighting a emerald lantern. Level 49 Firemaking is required to light it. If lost, the emerald lens can be recovered by speaking to the Cabin Boy who can be found slightly north-west of the Lunar Isle bank.
During the quest, the item is used to uncover markings on specific objects around the Lady Zay which have been jinxed by the Cabin Boy (uncovered during the quest).
This lantern cannot swap the emerald lens with bullseye lantern, as the sapphire lantern can.