Question: What does the locket Luce get look like?

Hey, everyone. My girlfriend really loves this series and she mentioned a necklace or locket the main character gets at some point. If I'm not mistaken she mentioned it opening with pictures inside. Anyway, her birthday is fast approaching and I was hoping to get a replica or a necklace similar to it for her for her birthday. But in order to do that, I need to know what it looks like. So can anyone here tell me?

Be aware that I'm not personally familiar with this series.