The overt, moral lesson is that "money can't buy happiness." Before finding the "Pearl of the World," Kino's family is poor but content. Their basic needs (food, shelter) are met. Kino and Juana have a network of friends upon whom they can rely.
The discovery of the gem changes all that; suddenly, Kino is no longer satisfied with having base needs fulfilled. He becomes more and more obsessed with the idea of how wealth can change their lives, thinking money will be able to ensure a better life for his family
But the pearl has insidious effects. It separates Kino from his people. When Juana asks, "Who do you fear?"
Kino searched for a true answer, and at last he said, "Everyone. " And he could feel a shell of hardness drawing over him.
Juana sees how the pearl is destroying her husband's integrity. She begs him to rid them of the supposed treasure; she foresees the horrific consequences: "It will destroy us," she cried. "Even our son."
Juana is right. It will take the death of Coyotito to shake Kino out of his money-tinged reverie. Steinbeck's moral seems to be that money, instead of happiness, frequently purchases misery.
The moral of the story is that things or objects are not what is important in a persons live. What is important is life itself. Also, family is important. When people have many riches, they want more. In this story, Kino, his wife and the baby were hardly noticed until Kino found "The Pearl." What did the rich people do when Kino's baby was near death, ignore the family because he didn't have any money. However, after the finding of the Pearl, Kino and his family had more friends than they knew what to do with. Of course, all the people truly wanted was the money from which the pearl would generate. To be a selfish person is not a good paradigm to have. It took the life of Kino's son to realize what he had and what he lost.