When Hermione had decided to fuck with Bellatrix Lestrange, she'd planned on taking a piece of jewelry. Some ostentatious necklace or tiara that she could wear to the next Death Eater party that she would be dragged to because Theo grew restless without her there. A piece that Bellatrix would immediately recognize as hers — as stolen — something that would drive the other woman insane with the knowledge that she'd waltzed right into her vault and taken whatever she wanted.