"The Pearl" is considered to be a parable; therefore, you would say that the main or central idea is actually the moral or message of the story. This moral is that people are sometimes so fixated on greed and becoming wealthy that they lose sight of what is important in life. This main idea or moral is evident in quite a few characters in the novella. Kino is so fixated on the idea that the pearl is going to make him rich that he ends up beating his wife and mistakingly killing his young son. The pearl buyers and the doctor are also so overcome by greed that they lie, cheat, and try to steal in order to get money. Finally, it is the character of Juana who shows an example of how people should act rather than the way that the novella's greedy characters do act.
John Steinbeck recounts the story of an indigenous native fisherman of Mexico in his non-fiction work, Sea of Cortez, and describes the tale of a man who finds a pearl of great value only to have his life ruined. In that brief telling, the story of The Pearl would seem to be mainly concerned with the ironies of material wealth versus non-material wealth (like emotional well-being, stability, family, etc.).
However, in the narration of The Pearl Steinbeck offers a pointedly different take on the meaning of the story.
“For it is said that humans are never satisfied, that you give them one thing and they want something more. And this is said in disparagement, whereas it is one of the greatest talents the species has and one that has made it superior to animals that are satisfied with what they have.”
This statement suggests that Steinbeck is less concerned with the notions of learning to value non-material forms of wealth and more concerned with ideas of relativity as regards an individual's perspective on his or her own limitations.
Kino finds a pearl of great worth in The Pearl and his life is ruined. This is the story parsed down to a single line. Yet this line does not communicate some of the essential elements of the narrative because it leaves out the fact that Kino was once denied health care for his ailing child because he is seen as being too poor. When he finds the pearl and becomes wealthy, materially, he begins to see greater possibilities for his family. He envisions schooling for his child and other things that can only be seen as credible and empowering aims.
Thus the central idea of the text is connected to the shifting sense of what is possible and also what is desirable. It is a story about changes perspectives and expanding horizons, while at the same time exploring the political/social limitations that govern life in the region and which serve to destroy a fledgling hope for a better life.