Symbolism, Imagery, Allegory
The pearls first appear by surprise. Wang Lung and O-lan are engaging in some husband–wife relations when he realizes that her breasts feel strange. Yeah, well, that's because she has a whole bunch of jewels hidden between them! How did he miss that?
Wang Lung immediately wants to use the jewels to buy land, but O-lan has a request, "'If I could have two,' she went on humbly, 'only two small ones—two small white pearls even... 'Pearls!' he repeated, agape. ‘I would keep them—I would not wear them,' she said, 'only keep them.'" (16.18) Aww, man. Every time we read that, we feel like we did when Bambi's mom died.
But then something amazing happens. Wang Lung is "moved by something he did not understand," and he actually lets O-lan choose two pearls (16.24). After that, Wang Lung thinks about the hidden pearls every now and then. She never talks about them or even takes them out to look at. They're just there, hidden, close to her heart.
The pearls get mentioned again when O-lan gives birth to the twins: “And when Wang Lung went into the inner room there was O-lan upon the bed with two new-born children, a boy and a girl as alike as two grains of rice. He laughed boisterously at what she had done and then he thought of a merry thing to say, ‘So this is why you bore two jewels in your bosom!'" (17.10).
It's kind of strange that the twins look totally different from their siblings, right? They're both pale and more beautiful than the other siblings put together. Since pearls are prized for being white and beautiful, it wouldn't be surprising if the ones that O-lan keeps have something to do with that, almost as if the pearls themselves had some influence on the birth on the twins.
The next time that we see the pearls, it's not such a happy occasion. Wang Lung has come to take them away from O-lan: "There is no use in keeping pearls for nothing"(19.68), he says. What he really means is there is no use in an ugly woman keeping them. So he takes the pearls and gives them to Lotus.
We don't hear much about the pearls after that, but when O-lan dies, Wang Lung remembers them. That's his only regret. "And out of his heaviness there stood out strangely but one clear thought and it was a pain to him, and it was this, that he wished he had not taken the two pearls from O-lan that day when she was washing his clothes at the pool, and he would never bear to see Lotus put them in her ears again" (26.85).
The pearls are the only things that O-lan has ever had as a reward; they symbolize the only time she has ever really felt appreciated. 99% of the time, she just works and works, and no one says thank you or anything. So when Wang Lung gave her the pearls, they were like a symbol of his appreciation for her—in other words, his love for her. They were something she could hold on to during all her suffering.
Maybe it's that love that allowed her to give birth to such beautiful children. When Wang Lung takes the pearls back, he thinks he's just taking pearls. But really, he's taking away his love for O-lan. It's no coincidence that the pearls go to Lotus, Wang Lung's new love.
And we know how that love works out.