From The Grey House:
Remembering how she felt when her mother died and gave back her memories, Natalia reeled the feeling in and clamped a hard mental hand on her control. Understanding gripped her. It was not just the necklace, but her will that made the hypnosis work. She could keep control of those she hypnotized without the necklace, but probably still needed it to get someone under her control. Knowing she could later retrieve her mother’s necklace that still lay in her jewelry box, Natalia made her decision. She opened her eyes, slipped off the necklace, and handed it to Zechariah.
Cat Stark 