The Diamond Necklace is a necklace that Freud obtained during the 2nd arc when he saved the British People from the attack of the DARK FORCES.
Freud obtained it from Namu during the time that he first met him. It is said that the DIAMOND NECKLACE protects its wearer from danger and is a powerful talisman of protection, it also drives out misfortune and other bad aura while worn.
During the 3rd arc, Freud gave it to Kimberly Ong because he believes that she might face a lot of danger and to avoid that he gave her protection. He knows that not all the times he can easily reach her and save her.