Symbolism, Imagery, Allegory
Gemma knows so little about her past and her family that when Alex gives her an old locket, she's thrilled. He's not supposed to give it to her (since it might spark her emotions and all), but he does so anyway because he feels sorry for her. When Gemma opens up the locket, though, she sees a picture of Stephan and immediately realizes who he is:
I didn't answer, sliding the tip of my finger to the man that stood beside her. "So who's this next to her?"
"That's my father." He gave me a funny look. "Why?"
I jumped to my feet, panic firing through me as wildly as the sparks did when Alex had kissed me. "Because he's the one I saw force my mom into the lake." (31.110-112)
Opening the locket is a bit like opening Pandora's box for Gemma. Suddenly everything clicks together in her head: Stephan is the man who pushed her mother into the lake and who keeps appearing in Gemma's nightmares accompanied by his army of Death Walkers. Yikes.
Even though Alex tries to convince Gemma that Stephan isn't the bad guy, once Gemma opens the locket, there's no going back. She knows that Stephan is up to no good—and nothing that Alex can say will convince her otherwise.