The Pearl by John Steinbeck is about desire. Kino the main character finds a pearl and he sees in the pearl a better life for himself, his son, and his family. However, as the story progresses, he only sees misery and even death. It is the story of the illusory joy of things.
In our society, there are two things that people usually see as their "pearl" of joy.
First, many people believe that if they have a lot of money, they will be happy. They believe that wealth is the cure all of all their problems and miseries. However, all you need to do is to look at wealthy people and you will conclude that wealth does not necessarily bring happiness.
Second, many people believe that to have a relationship is their "pearl." However, this too, is illusory. We need to keep in mind that half of all marriages end in divorce. The point is that all "pearls" can be illusory. It is about our hearts not what we have.