Cuff reports that the Indians still seem innocent, though it is clear that they have come to steal the Moonstone. Cuff also reports that Rosanna bought a piece of long cloth of a particular quality at the linen draper's on Thursday. Cuff has concluded the material was to make her a new nightgown.
It is two o'clock, and the carriage for transporting Rachel to Frizinghall pulls up to the house. Cuff speaks with her footman, explaining that he must secretly pick up a man of Cuff's onto the back of the carriage to follow Rachel. Rachel steps outside and kisses her mother briefly, saying, "Try to forgive me, mamma." Rachel ignores Cuff when he warns her that she is impeding his investigation by leaving. She also ignores Franklin, who wishes her goodbye. As she drives away, Franklin breaks down and resolves to leave by the next train.
Cuff looks for the police officer Joyce, who was ordered to watch Rosanna. Joyce admits he lost track of Rosanna an hour ago. Cuff dismisses him from the case. Cuff determines that Nancy, the kitchen maid, last saw Rosanna leave with a letter in her hand. She gave the letter—addressed to Cobb's Hole—to the butcher's man to mail. Cuff decides to go to Frizinghall to wait for Rosanna to contact Rachel there.
Another servant reports having seen Rosanna running toward the beach. Cuff leaves for the Shivering Sand. Soon after, he sends back for Betteredge, asking for one of Rosanna's boots. Betteredge brings the boot himself, and Cuff matches it to a footprint on the sand. The boot fits. The footprints lead to a rocky ledge called the South Spit. There are no footprints returning from the ledge, and it becomes clear that Rosanna has met with a fatal accident. Betteredge, choked up, guesses that she has committed suicide. A servant boy runs up with a note found in Rosanna's room. The note thanks Betteredge for his kindness and asks for his forgiveness when he next sees the Shivering Sand—"I have found my grave where my grave was waiting for me."
Back at the house, Lady Verinder is distraught over Rosanna's suicide. She blames Cuff.
Cuff meets with Penelope and Betteredge. He questions Penelope about Rosanna's death, and Penelope guesses that it was for love of Franklin. The three agree not to tell Franklin of his possible part in Rosanna's suicide.
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.
Cuff's presentation to Lady Verinder of his suspicions of Rachel and her motives centers on a conjecture of private debts that Rachel might owe. Though Lady Verinder remains unconvinced, and indeed, we will see that Rachel has no debts, the dynamic of debt will work throughout The Moonstone as motivation for theft of the diamond. The diamond can be used as potential surety to a moneylender in lieu of cash. This English dynamic of economic debt allows us to see the transformation of the diamond from its spiritual significance as part of a religious idol, to sentimental significance as a family heirloom, to economic significance as a commodity to be bought or sold.