Peeta's pearl

"Sometimes when I'm alone, I take the pearl from where it lives in my pocket and try to remember the boy with the bread, the strong arms that warded off nightmares on the train, the kisses in the arena."

- Katniss Everdeen

In Catching Fire, while diving for seafood to eat for dinner, Peeta Mellark finds a pearl in an oyster and gives it to Katniss Everdeen as a gift when he talks to Katniss about how he will protect her. When he found it, he told Finnick Odair that when enough pressure is placed on coal it will turn into pearls. This is an inside joke between him and Katniss referring to Effie Trinket's mistaken pronouncement before their last Hunger Games, which confused pearl creation with the creation of diamonds. Katniss closely guards his gift, thinking that her mother and Prim will surely know to return the pearl to Peeta if she dies protecting him, which she's positive she will.

She later brings it to District 13, waking up in the middle of the night and looking at it. She keeps it with the spile and the other items she received in the arena, like Peeta's locket. She then takes it on her mission to assassinate Snow. Throughout Mockingjay, Katniss closely guards the pearl, keeping it in a silver parachute in a dresser drawer. Later, Katniss' mockingjay uniform has a pocket in which Katniss stores her pearl throughout the Battle of the Capitol. The last time we hear the pearl being mentioned is when Katniss drops the keys to unlock Peeta's handcuffs in her pocket and she hears them clinking together. After that the pearl (along with the spile and the mockingjay pin) was returned to Katniss and is in her house in the Victors' Village.