The title is important as it refers to the central symbol and theme of the book. The great pearl which Kino finds represents his longing and dream to better his life and achieve wealth and happiness – a universal human trait.
When Kino first spies the pearl it is described in richly lyrical terms:
...there it lay, the great pearl, perfect as the moon. It captured the light and refined it and gave it back in silver incandescence. It was as large as a seagulls' egg. It was the greatest pearl in the world.(chapter 3)
However, there is also danger in this pearl which Kino does not realise at first. The ominous signs are there from the beginning in the narrative, with the observation (in Chapter 3) that this pearl hails from the same bed which gained great wealth for the Spanish in the past centuries, enabling them to colonise and oppress Kino’s people.
The pearl certainly changes Kino’s life, but ultimately not in the way he hoped. It attracts malice and envy, drives him from his community and in the end it cannot save the life of his little son.
The pearl can also be seen to represent Kino, the central character; he certainly comes to view it this way.
'This pearl has become my soul,’ said Kino. ‘If I give it up I shall lose my soul.’ (Chapter 5)
By the end, with his whole world in tatters, the pearl comes to appear ugly to him, reflecting his own deterioration.
It was grey, like a malignant growth. And Kino heard the music of the pearl, distorted and insane. (chapter 6).
There is a moralistic slant to the story; that sudden good luck or fortune brings its own problems, and that the desire for material wealth creates greed and envy. It also hints that human happiness is itself extremely transient, perhaps just an illusion. The pearl is the main symbol of all this.
It seems Kino is soundly punished for his presumption in leaving behind the former conditions of his life, for daring to aspire to better things, but overall it does seem rather unjust. However, there is a deep sense of pessimism ingrained in the book, the sense that the universe is essentially hostile to all human endeavour:
(Kino) knew that the gods take their revenge on a man if he be successful through his own efforts. (Chapter 3)
Ultimately, Kino’s plans for a better life for himself and his family bring utter catastrophe.