What is the theme of The Pearl by John Steinbeck?

John Steinbeck's novella The Pearl was published in 1947. It tells the story of a poor family, father Kino, mother Juana, and child Coyotito, who try to live with the consequences of Kino's discovery of a great pearl. The pearl should make them rich (at least in terms of the context in which they live), but its value leads to all manners of unexpected problems and one tremendous tragedy.
Students often have trouble determining themes to a work. There is frequently more than one theme to any work as long as a novel, or even a novella. To find a theme, the student should ask themselves: What is the main message that the writer wants to get across? If I could boil this work down to one or two sentences, and still maintain its spirit, what would those sentences be? Make sure that the theme you formulate is not a summary of the story, or an event from the story—it has got to be the writer's central, universal message, and it must be stated in your own words, not the words of the author. 
For a story like The Pearl, I think you have to take the ending into account when determining the theme. As the family tries to escape persecution (because of others' greed for the pearl), their infant son is killed. At that point, they give up their journey and return to their hometown. Strangely, however, they are not broken--they are surprisingly strong.
When they reach the shore, Kino takes out the giant pearl and looks at it:

Evil faces peered from it into his eyes, and he saw the light of burning . . . and the pearl was ugly; it was gray.

These lines signify how Kino has changed. Character change is often a strong clue to theme. Then he flings the pearl into the ocean, where it settles out of sight under the sand on the ocean floor.
So, with this in mind, what is Steinbeck's central message? He has shown us a young family that makes a great discovery that should lead to happiness and prosperity—but it leads only to trouble and the ultimate heartbreak. The pearl, a symbol of beauty throughout the world, has become ugly to him, and he chooses to cast it away.
It should be noted that, throughout the ordeal, the family has been able to stay together and support each other. Several other family members have stood by them without being consumed by the greed that has afflicted so many others in the story.
With all of this in mind, I would say the theme is something along lines of:
The pursuit of happiness or fulfillment cannot be tied to a material object or the search for wealth, because those things are subject to change and corruption—we must seek happiness in our relationships with the important people in our lives.