The Emerald of Ekron made its first appearance in Adventure Comics #352.
It was the actual eye of the cosmic entity, Ekron but now is in the possession of the Emerald Empress and the source of her power, giving her such abilities as flight, super strength, ability to survive in the vacuum of space and the power to emit blasts of energy.
Recently the Eye has been seen in the possession of Lobo during 52 Lobo kept it in a box where it would remain under his supervision. Soon there after Starfire used the eye to stop a bunch of beings from destroying an entire space sector. After this even Lobo reveals some of the history behind the Eye. Apparently there was an Emerald Head of Ekron who was a member of the Green Lantern Corps. Ekron was driven insane by the destruction of the sector under his command - much like Hal Jordan and Coast City minus the evil influence of Parallax. The Eye itself is an early design of the power rings worn by all Green Lanterns just with less functionality.