Dragoon: Base Attack increase. Increase % and amount based on Star.
Overlord: During it's duration any buff skill casted will last longer depending on the Star. (1star, 10 second increase. Goes up with higher star necklaces).
Setinels: Block X Number of Elga's attacks based on Star. (1star = 1 attack) (Currently not working properly according to Stormhaven.)
Destroyer: From a test in duel by a friend of mine; It makes a shield that blocks all attacks for a limited amount of time. (Higher Star means Longer Duration)
Invokers: % Increase to Base Magic Attack based on Star.
Sorcerors: Increases the duration of Debuffs to foes. Duration increase based on Star
Savages: Aim Buff based on Star.
Ninjas: Evade Buff based on Star.
Summoners: Unknown atm.
Twin Fighters: Unknown atm.
Hope this helps you consider what you'll be going for when farming Elga necks. Will try to update list as new stuff is revealed.
Edited by Coolsam, 28 March 2013 - 09:04 AM.