The silver necklace is a quest item used in the Murder Mystery quest. It is obtained from a barrel in Anna's bedroom in the Sinclair Mansion during the quest. It has Anna's fingerprints on it and can be used to figure out if Anna is the murderer.
This item will disappear if a player has it in their bank or inventory upon finishing the quest. It is a unique cosmetic item, so if a player wishes to keep it, they must drop it before finishing the quest and pick it up again afterwards. It cannot be obtained again after the quest is finished.