What did Elizabeth deposit in exchange for the magical comb The Grimm legacy?

In "The Grimm Legacy" by Polly Shulman, Elizabeth deposits a family heirloom, a silver locket, in exchange for the magical comb. The comb has the power to summon the spirits of the dead, which Elizabeth seeks to help her in her quest. This act illustrates her willingness to sacrifice something precious for the chance to gain knowledge and power.