Intro
The Pearl is a parable: as Steinbeck notes at the beginning, the story is deliberately simple because its purpose is to get down to basics and make its point through allegory.
The story is a cautionary tale about a fisherman, Kino, who lives a peaceful, rustic life with his wife, Juana, and baby son, Coyotito, on the outskirts of a Mexican town called La Paz—until one dark day, a stray scorpion stings little Coyotito and the family discover the flaws in the healthcare system (that's right, back in 1927). To try to scrounge up the dollars for the doctor, Kino takes to the sea to find a pearl, and—lucky day!—find one he does. But it is really so lucky? While Kino is dazzled by the pearl and its promise, it ends up bringing nothing but pain, misery, and, above all, greed.
The Pearl focuses on some key cultural studies topics: Kino's people are culturally coded as "other" and inferior within the novel's colonial setting, which Steinbeck critiques by depicting the exploitation of Kino and his family by the Scrooges who will stop at nothing to grab Kino's shiny pearl. Clearly, capitalism comes in for some major flak, with Steinbeck showing La Paz to operate through materialism, avarice, and class-based division (yup, the Frankfurters would be pissed). Like the pearl, then, La Paz is symbolic of a much wider system, the injustices of which Kino learns the hard way as his life starts to spiral out of control. Becoming a victim of not only the envy and brutality of others but his own aggressive impulses, Kino realizes that the pearl is more trouble than it's worth. All that for the soft tissue of a mollusk? By the end of the novel (duh—spoiler alert!) it's clear what he must do:
Quote
And when they came to the water's edge they stopped and stared out over the Gulf. And then Kino laid the rifle down, and he dug among his clothes, and then he held the great pearl in his hand. He looked into its surface and it was grey and ulcerous. Evil faces peered from it into his eyes, and he saw the light of burning. And in the surface of the pearl he saw the frantic eyes of the man in the pool. And in the surface of the pearl he saw Coyotito lying in the little cave with the top of his head shot away. And the pearl was ugly; it was grey, like a malignant growth. And Kino heard the music of the pearl, distorted and insane. Kino's hand shook a little, and he turned slowly to Juana and held the pearl out to her. She stood beside him, still holding her dead bundle over her shoulder. She looked at the pearl in his hand for a moment and then she looked into Kino's eyes and said softly: "No, you."
And Kino drew back his arm and flung the pearl with all his might. Kino and Juana watched it go, winking and glimmering under the setting sun. They saw the little splash in the distance, and they stood side by side watching the place for a long time.
Analysis
This is a classic "careful what you wish for" situation. These final passages show how Kino and Juana's simple, contented life has been shattered by the evil forces of the pearl. After all that the couple has endured—being exploited, attacked, hunted, and finally losing their son—it's safe to say that the pearl has lost its luster.
These lines are key because Juana predicts early in the tale that the pearl will bring only misery, but Kino insists otherwise. However, now that he's found out the truth the hard way, it is emblematic that he's the one who must cast the pearl back into the sea.
Plus, that the pearl itself is now described as "grey and ulcerous" highlights the core theme of Steinbeck's novel: the pearl being a reflection of human nature. In the same way that Steinbeck invites the reader to read their own life into the story, the pearl's transformation symbolizes the destruction and misery that have been stirred up as a result of the pearl—and, we might add, the wider system of cutthroat capitalism. You can imagine Marx saying "told you so."
From a cultural theory perspective, then, the relevance of this passage (not to mention the novel as a whole) comes from its use of allegory to highlight wider social issues; most of all, the dangers of capitalism seem as threatening as that shinily evil pearl.