Could have been made by the Vikings (or whomever they dragged along to do such work), and sold or traded at one of the interfaces, then wound up deep off into a non-Viking area. Or, it could have been taken as booty, and lost or concealed at some point later. So IOW yeah, I do appreciate the validity of your comments. :’)
Or, there was an unplanned event, a ship wreck or fatal illness that caused the party to take shelter in the cave. The necklace owner died and was left in the cave