The novel is named after the "devilish Indian Diamond" (126.96.36.199), and the disappearance of the Moonstone is what sets off the whole mystery. Because the diamond is so central, there are a lot of theories as to what it might symbolize. Let's go over some of the theories…
The diamond is called the Moonstone because it looks like the moon – it has a yellowish glow to it. It also changes depending on the light. The moon changes, too, depending on the time of month. Changeability is an important idea in the novel, which may be why Collins wanted the diamond to be associated with the moon.
Some people think that it symbolizes India. After all, the diamond comes from India and is sacred to a certain Hindu sect. This could make a certain amount of sense: the diamond is stolen from a Hindu priest by an Englishman, just as India is taken over by British colonial forces. But, by the end of the novel, the Indians reclaim the Moonstone and take it back to India. Could this mean that that Wilkie Collins wanted to suggest that the British colonial project in India was doomed to failure, and that the Indians would eventually regain self-rule?
Of course, if the Moonstone represents India, what are we to make of the "flaw" at the center of the diamond (188.8.131.52)? Perhaps the flaw represents a fundamental flaw in England's perception of India, or a "flaw" in their colonial project. Or, perhaps, the flaw is Collins's subtle suggestion that India, itself, has some fundamental flaw.
Still other critics believe that the diamond represents femininity, or even virginity. It wasn't uncommon in poetry to talk about a woman's chastity, or virginity, as a "jewel" of tremendous worth. So that would make the theft of Rachel's "jewel" – especially from the supposedly private sanctuary of her boudoir – into a kind of symbolic rape. But if the diamond represents Rachel's virginity, what would the flaw at the center of the jewel suggest?
There are obviously a lot of possible interpretations of the diamond. What do you think?