Necklace | JoJo's Bizarre Adventure | Mr. President's Key

Mr. President's Key
An ornate key in which is embedded a transparent marble. On the marble is engraved instructions for finding Coco Jumbo in Naples's train station, in which it fits a cavity on the turtle's shell. The key is heavily linked to Mr. President, as it activates the Stand and taking it out of the shell forcibly ejects any living being from the room inside Coco Jumbo.
(from JoJo's Bizarre Encyclopedia - jojowiki dot com)

JoJo's Bizarre Adventure (ジョジョの奇妙な冒険, JoJo no Kimyō na Bōken), often shortened to JoJo or abbreviated as JJBA, is a shonen and seinen manga written and illustrated by Hirohiko Araki. JoJo's genre spans Action, Adventure, Supernatural, Thriller, Comedy, Tragedy, Mystery, Slice of Life, and Horror. (from JoJo's Bizarre Encyclopedia)

The nickname "JoJo" is an abbreviation of the name of the Joestar family protagonists (Jonathan, Joseph, Jotaro, Josuke, Jolyne, Johnny, Jodio) in each of the eight manga story arcs whose adventures it chronicles.