The Necklace is an object that originally belonged to Tara Desai until her murder. The necklace somehow ended up being worn by Regina Crane 4 1/2 years later, and she was murdered 6 months after that upon the return of Danny Desai, who then retrived it supposedly the day after after "finding it in his locker". He then kept it hidden in his house for weeks after Regina's murder. It is unknown what the necklace is or what power it has. The police department were frantically looking for it as a piece of evidence about who killed Regina.
In The Fest And The Furious, Karen Desai was seen by Rico throwing the necklace into the lake.
In We Need To Talk About Danny, Eddie managed to receive the necklace from the lake and shows up at the home of Chief Masterson with it in an evidence bag.