I think the most obvious place to start in answering this question would be to consider how the pearl changes through the story. The pearl can be viewed as a symbol of avarice in the tale. Note how every character tries to manipulate Kino into gaining it. However, when Kino first finds it, it is described as "perfect." It has a "secret melody," that is depicted as "clear and beautiful, rich and warm and lovely, glowing and gloating and triumphant."

However, at the end of the tale, after Kino has lost his son, note how the pearl is described and how it has transformed:

He looked into its surface and it was grey and ulcerous. Evil faces peered from it into his eyes, and he saw the light of burning. And in the surface of the pearl he saw the frantic eyes of the man in the pool. And in the surface of the pearl he saw Coyotito lying in the little cave with the top of his head shot away. And the pearl was ugly; it was grey, like a malignant growth. And Kino heard the music of the pearl, distorted and insane.

The pearl, which had started as a symbol of hope and of dreams and new promises for Kino, now is shown to be the source of all evil and wrongdoing that has visited itself on Kino and his family. Note in particular how the music of the pearl is now described as "distorted and insane," reflecting the impact of avarice on mankind and how it is capable of turning men into something evil.