Millicent doesn't know where the necklace came from? Let's hope that Zabini tries to sell it off as Goblin-made and many years old, only to have the Goblins turn on him when it turns out to be Muggle-made and recent! Costume jewelry, you know.
Poor Millicent. Zabini really is a cad. Though it also goes to show that this prettying up a person beyond recognition is as likely to backfire as anything.