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.
Mr. Betteredge employs animal metaphors and verbal irony to describe the "wicked Colonel" John Herncastle in Chapter 5 of Period 1, implying to the reader that he is vicious, instinctual, and fierce:
They are very strict in the army, and they were too strict for the Honourable John. He went out to India to see whether they were equally strict there, and to try a little active service. In the matter of bravery (to give him his due), he was a mixture of bull-dog and game-cock, with a dash of the savage. He was at the taking of Seringapatam.
There is a sense of insincere, comedic restraint in Betteredge's descriptions of Colonel Herncastle in this passage. Betteredge calls Herncastle "one of the greatest blackguards that ever lived" just before this. However, he goes on here to refer to him by the sarcastic nickname "Honourable John." Of course, someone who is a "blackguard" is not honorable at all, which is why the verbal irony of this misnomer is so effective. This sarcasm continues in his description of Herncastle's time in India as a way of seeing if "they were equally strict there": the idea that the British Army would be more lenient in some places is an unlikely one.
In saying that the Colonel has "a dash of the savage," Collins uses the metaphor of animals trained to fight to death in betting arenas to characterize Herncastle. "Bull-dogs" and "game-cocks" were chosen for their savagery, and so, he implies, Herncastle is savage by nature.
The word "savage" here is also a pejorative reference to the native Indian population that Herncastle goes to India to try and subdue. The "taking of Serangitapam" was a famously violent suppressive military action by the British in colonial India. As Collins immediately refers to this battle after Betteredge's animal metaphors, it's unclear whether he means the "dash of the savage" he says Colonel Herncastle has in place of "bravery" is an Indian or an animal trait. Given the largely negative depictions of Indian characters in this novel, it seems likely to serve both purposes.
