What is interesting about the pearl, which is clearly the central symbol of this novella, is that the way it is depicted changes during the course of the novella, and in addition, that these changes relate to the changes that Kino as the central protagonist experiences. Initially, for example, the pearl is seen as a stroke of luck. However, Kino then begins to look at the pearl as if it were the answer to all of his prayers and the means by which he will be able to educate his son and free him from the vicious cycle of poverty and dependence that has plagued his people for so long. Note what he says about the pearl:
In the pearl he saw Coyotito sitting at a little desk in a school, just as Kino had once seen it through an open door. And Coyotito was dressed in a jacket, and he had on a white collar and a broad silken tie. Moreover, Coyotito was writing on a big piece of paper. Kino looked at his neighbors fiercely. “My son will go to school,” he said, and the neighbors were hushed. . . .
Kino’s face shone with prophecy. “My son will read and open the books, and my son will write and will know writing. And my son will make numbers, and these things will make us free because he will know—he will know and through him we will know. . . . This is what the pearl will do.”
What is key to spot in this quote is the violence of Kino's obsession with the pearl. This is the moment where he sets out his plan for how he will use the pearl, and his words speak of how violently he clings on to this vision, even in the face of the threats and danger that his family face as a result. This is the beginning of Kino's change from a loving husband and father to a man who becomes overwhelmed by the greed of the wealth that he now has.
The pearl therefore seems to be something of a neutral object that is shaped by the thoughts and feelings of Kino and his own transformations. If the pearl in the final lines of the novel is described as being "grey and ulcerous," and evil, this is only because it has been made evil by the greed of man. In a sense, the pearl therefore acts as a mirror of the men that yearn for it.
This is a great question. There will be differences of opinion. In my opinion, the pearl is neutral. It is neither good or evil. It is simply a thing that people accord value. What is evil or good is the hearts of people. The pearl only causes people to decide between good and evil. Let me explain.
Kino obviously does "evil" things on account of the pearl, even if some of his actions are understandable in view of his sick son. But he goes too far. He kills a man (perhaps two) and hits his wife.
Juana reacts differently. She wants to throw the pearl away. She is not in its power, because her heart is different.
These two points show that the pearl is not the source of good or evil. What matters is the heart. Steinbeck shows this. Here is a quote of what Kino sees in the pearl.
"In the pearl he saw Coyotito sitting at a little desk in a school, just as Kino had once seen it through an open door. And Coyotito was dressed in a jacket, and he had on a white collar and a broad silken tie. Moreover, Coyotito was writing on a big piece of paper. Kino looked at his neighbors fiercely."
This quote clearly shows that what leads Kino to do what he has done is his desires.
It is an interesting question. The pearl is an evil because it only brought troubles than goodness. It did what a scorpion did not do; it caused the death of the baby. Juana becomes conviced that the pearl is evil and it will destroy their life.