Cuff returns to Lady Verinder, who offers to pay him and dismiss the case. Cuff refuses payment, saying that he has not completed his duty—solving the case.
Cuff reveals his suspicions of Rachel to Lady Verinder. Though Lady Verinder admits that she has no more information from Rachel than Cuff, she counters that Rachel is "absolutely incapable" of committing the crime. Cuff suggests that Rachel has private debts that she will use the Moonstone to pay off. Cuff reviews all of Rachel's suspicious and unhelpful behavior throughout the investigation. Cuff reviews the evidence against her and the evidence pointing to Rosanna as her accomplice. Lady Verinder still refuses to believe him.
Cuff proposes to keep watch on Rachel. He also proposes to plant a servant under his employment in the household and to track a certain London moneylender with whom Rosanna was acquainted in her thieving days. Lady Verinder declines these proposals. As a final suggestion, Cuff proposes that he go to Rachel and tell her suddenly of Rosanna's death to see the effect the news has upon her. Lady Verinder agrees to this strategy, but she insists that she be the one to tell Rachel.
Wilkie Collins is somewhat unique as a Victorian novelist in his attention to and depiction of the practice of suicide. Rosanna Spearman's love for Franklin Blake and subsequent suicide are here depicted sympathetically and tragically (unsentimentally) realistic. Further on in the novel, Rosanna's story will become clear through a letter she leaves before her death. It has been remarked that the character of Rosanna is more vividly rendered than even Rachel Verinder. Rosanna's deformity, her tragic quality, her sensuality, and her idealistic ignorance of class division make her a distinctive character in a novel in which women don't speak often and therefore do not have a distinctive presence.
Rosanna's suicide reveals the foreshadowing contained in her earlier comment in Chapter IV at the Shivering Sands, "sometimes, Mr. Betteredge, I think that my grave is waiting for me here." Rosanna's suicide, to some extent, has come to be associated with the injustices of class difference. Her love for Franklin was deemed foolish by all, not simply because Rosanna is homely and deformed, but because she is a servant and he a gentleman. Collins subtly outlines these connections in part through the setting of the Shivering Sand. Also in Chapter IV, Rosanna described the quicksand almost in terms of class oppression, "It looks as if it had hundreds of suffocating people under it—all struggling to get to the surface, all sinking lower and lower in the dreadful deeps!" The place of Rosanna's death thus becomes synonymous with the tragedy that inspires her death. Collins uses the weather, as well, throughout the novel to enhance mood. Thus when Betteredge follows Cuff down to the Shivering Sands to discover the evidence of Rosanna's suicide, a black storm gathers.
The agitation of the household in these recent chapters can now be seen as distress at the respective departures of Rachel and Rosanna. Rachel has merely left for Frizinghall, while Rosanna has killed herself. Yet these unnatural behaviors on the part of female characters creates a frantic atmosphere among the characters remaining at the Verinder house. This parallelism also reinforces the position of Rosanna as Rachel's counterpart in the outcast world. Several of the socially prominent characters in The Moonstone will have marginal counterparts, such as Franklin Blake's counterpart of Ezra Jennings later in the novel.