The Moonstone is a novel packed with situational irony, particularly surrounding the theft and discovery of the diamond from which it takes its title. Collins's narrative is full of foreshadowing, which is often subverted by situationally ironic twists and turns. This contributes to its effectiveness as a detective novel, as it keeps the reader guessing throughout. The irony is a key part of the novel's mystery plot, as it relies on mischance and unexpected happenings to move forward. Getting to the facts takes a long time. Notably, the section of the novel entitled "The Discovery of the Truth" only comes after two other large sections, and it takes eight chapters to resolve itself.
It seems obvious at the beginning of the book who has stolen the Moonstone from Rachel Verinder's bedroom, but that conclusion is quickly proven to be a red herring. Characters like Rosanna Spearman appear to the reader to have the means, motive, and action to be the thieves, but are then revealed to have actually been noble and self-sacrificing. Throughout the novel, Rosanna expresses fear and loathing of the murky and uncanny depths of the Shivering Sand, where things go to be forgotten. She is desperate to be noticed by Blake and fears being overlooked or forgotten about by him more than anything else. This makes it situationally ironic that Rosanna dies deliberately (and unexpectedly for the reader), doing the thing she fears most, "losing" herself to the quicksand. Further, it's only when Blake goes to the Sand and finds her confession that he really "notices" her, even though she's long lost under what she previously called its "horrible," "sucking" surface.
After many more ironically plausible misdirections by Collins, the novel eventually reveals its crowning irony. In a book full of deliberate crime and false accusation, it transpires that the person who actually stole the Moonstone from Rachel's room didn't even know he was doing it at the time. This situationally ironic twist is compounded by the fact that Godfrey Ablewhite—whom Rachel agreed to marry because she believed Franklin Blake was the thief—was actually himself the perpetrator. There's another ironic factor at play here, too: Blake "stole" the Moonstone in order to stop it from being stolen in the first place. Collins's plots are densely interwoven, relying heavily on these moments of situational irony to provoke emotion and keep the plot moving apace.
Another one of many examples of situational irony is the case of the Three Indians, men who have been tasked with finding and retrieving the Moonstone from its captors. These characters are framed as being possible, if not very likely, perpetrators of the theft in the beginning of the novel. However, Collins later makes it clear that the diamond was actually stolen first from their religious sect. The very fact of the diamond's abduction from India and its possession by Herncastle, an Englishman, is an ironic nod to the theft of Indian resources from British colonies. The Three Indians don't steal the Moonstone from Rachel, but it was originally stolen from them.
