A few symbolic things in this novel are The pearl, plot, characters, the ocean, and the child, but there is much more.
Kino is 3rd generation as a pearl diver, and the uncertainty of the ocean reinforces the futility of ever doing better. The scorpion represents more uncertainty with the sudden presence of evil coming into their lives. It attacks the innocent child, and to save her, they need anti-venom. Anti-evil to save an innocent child adds blame and irony to the resulting problems of good fortune. Even when the need for the anti-venom goes away, a different type of evil from those pursuing Kino and his wife kills the innocent child with a stray bullet.
The Pearl is more significant than any previous one and suggests that this low-income family's fortunes can change. When it comes time to toss the pearl back into the ocean, the pearl's great value emphasizes that well didn't come from potential wealth.