Symbolism, Imagery, Allegory
At the soup kitchen, Nikki sees a bracelet on Mary's wrist and notices that it seems kind of out of place:
As she reached for the tray, an old silver bracelet slid down her arm and settled on her wrist. It was the only jewelry she wore and looked heavy on her frail wrist. It must've been an heirloom or something. (8.4)
But when Nikki asks about it, Mary gets secretive: "She deftly moved her hands under the table, a reflex action" (8.28). Mary says the bracelet was passed down through the women of her family, but then she won't say anything more about it. So clearly there's some secret significance to the bracelet. Right before the Tunnels claim her, we hear the truth from Mary:
The bracelet holds a secret the Daughters of Persephone have been protecting for centuries […] By telling you that, I've betrayed all of my ancestors. (29.41)
It takes a little digging, but Nikki and Jack figure out that the bracelet contains ancient Egyptian hieroglyphs on how to kill an Everliving. In other words, this bracelet is like a loaded weapon.
Mary takes the bracelet from her mom, mostly wanting to get her attention, but eventually decides to give it to Nikki, who's a total outsider. Why? Maybe Mary has realized that the Daughters of Persephone are being exploited by the Everlivings and she wants to turn the tables. Or perhaps Mary wants revenge against Cole's group of Everlivings, who are the ones who took her to the Feed. We never really know.
Whatever Mary's reasons for giving Nikki the bracelet, the jewelry signifies the secrets of both the Daughters of Persephone and the Everlivings. This reflects how in the book's plot, Nikki—and then Jack and Will—learn about the Daughters of Persephone and the Everlivings. The bracelet getting passed around symbolizes knowledge getting passed around… for better or for worse.