The third narrative of this section is by Franklin Blake, and it begins when he was called back from his travels in the East in the spring of 1849 with the news that his father had died and that he was heir to his father's fortune. Franklin confirms the thought from Betteredge's narrative that he was heartsick at Rachel's treatment and thus had left England. On arriving back in England, Franklin feels for Rachel as much as ever and finds that she is living under the care of her aunt, Mrs. Merridew. Franklin goes to see Rachel, but she declines to see him. Franklin and Bruff are as puzzled as ever at Rachel's cold behavior. Franklin resolves to take up the case of the Moonstone, in hopes of discovering the secret of her feelings. He leaves at once for the Verinder house in Yorkshire and is approaching Betteredge by sunset.
Betteredge invites Franklin in, but Franklin refuses his invitation, explaining that he doesn't want to enter Rachel's own house against her will. Betteredge is confused and distressed to hear that Rachel is still not speaking to Franklin. But Betteredge refers Franklin to a spare room for rent at Hotherstone Farm, near the Verinder house. As they walk toward Hotherstone Farm, Betteredge asks Franklin the reason for his visit. Franklin announces his intention to recommence the Moonstone investigation in order to win Rachel back. Betteredge informs him that he may begin by fetching the letter addressed to him from Rosanna Spearman, which Limping Lucy Yolland holds. Franklin wants to go to Cobb's Hole tonight, but Betteredge tells him it is too late. They agree to go early in the morning.
Franklin picks up Betteredge, who has lapsed into "detective fever" again, in the morning on the way to Cobb's Hole. At the Yolland's house, Lucy hears Franklin's name with contempt and gets Rosanna's letter for him. She beckons Franklin outside and stands looking at him with disgust, saying, "I can't find out what she saw in his face Oh, my lost darling! what could you see in this man?" Lucy asks Franklin if he is not remorseful. He is confused and says no. She thrusts the letter at him and leaves.
Franklin opens the envelope and finds a letter and a slip of paper from Rosanna directing him to follow the directions in the letter for an explanation of Rosanna's curious behavior toward him. The letter contains directions to a hiding place in the Shivering Sands.
Franklin and Betteredge go to the Shivering Sands and wait for the tide to turn. When it does, Betteredge leaves Franklin to explore the hiding place alone. Franklin finds the tin case, held in the quicksand by chains. Inside is a linen nightgown and a letter. Franklin pockets the letter and examines the nightgown, which has a smear of paint on it. He remembers Cuff's statement that the owner of the stained nightgown was also the diamond thief. Inside the nightgown is the name of the owner: Franklin Blake—"I had discovered Myself as the Thief."
Franklin is shocked, having no knowledge of having taken the diamond. Betteredge takes Franklin home. Franklin begins to read Rosanna's letter out loud.
Rosanna confesses that she loves Franklin. She briefly relates her personal history—she was forced into thievery as her father left and her mother had no money. Her life was not hard to bear until she was "taught to feel [her] own degradation" at the reformatory. Lady Verinder eventually took her in. She recalls the morning she first saw Franklin on the dunes and fell in love with him, though she knew that he would never love a servant. Rosanna developed a hatred for Rachel, whom Franklin did love.
Rosanna speaks of the morning after the diamond was stolen. Penelope, who was there when Franklin and Rachel finished the door, told Rosanna that the paint on Rachel's door had to have been smudged during the night. Tidying up Franklin's room later in the morning, she found his nightgown on the bed with the paint on it. Thinking that the nightgown was evidence only of illicit intimacy between Franklin and Rachel, Rosanna determined to hold on to it, perhaps to use against Rachel.
Here, Franklin stops reading, with mixed feelings. He now regrets his callous treatment of Rosanna, but he finds himself becoming angry with her as he reads on. Betteredge continues reading.
Rosanna writes that she decided to make a replacement nightgown, which she did overnight and replaced the next day. After the servants are questioned and the details of the theft laid out, Rosanna realized that Franklin was in Rachel's room to take the gem, not to see Rachel. She resolved to shield Franklin and went in the library to tell him of her resolve but was interrupted by Betteredge. Rosanna relates how she herself came under Cuff's suspicion next and bore it all for Franklin.
Here Betteredge stops reading Rosanna's letter and asks Franklin if he now remembers anything more about the night the diamond was stolen, but he doesn't. Franklin has already resolved to see Bruff for advice and to try to get Cuff out of retirement.
They are interrupted by a knock on the door. It is a strange-looking man with a "complexion of a gipsy darkness," a wrinkled face, and hair which had gone white on the sides, but remained black on top. He gives Betteredge a list and leaves. Betteredge explains that he is Ezra Jennings, Mr. Candy's assistant who has been tending to the sick as Mr. Candy has never recovered from his illness.
With the return of Franklin Blake to England, we can see that the investigation of the missing diamond takes on more urgency. Franklin has been the driving force of the investigation all along—first because he cared so much about Rachel and wanted to find her diamond and now because he equates the mystery of the diamond with the mystery of Rachel's cold treatment of him. Franklin is also the driving editorial force behind the assembled narratives of the diamond, and now we see why: Franklin is trying to clear his own name of suspicion in the theft of the diamond.
In an unusual plot twist, Wilkie Collins has the thief named just a little over halfway through the novel. Yet Franklin's guilt is still not apparent, for the mere fact that he does not remember committing the crime and believes himself innocent. We are never meant to distrust Franklin's assertion that he does not believe himself to have committed the crime, because we have been conditioned with reliable narrators. We may not have agreed with everything that Betteredge, Miss Clack, and Mr. Bruff asserted, but we never doubted the fact that they believed their assertions themselves and that they would not give false testimony. The possibility still remains here that Franklin has been framed, and Rosanna would stand as a likely agent of this framing. Franklin shows himself willing to believe this possibility, given Rosanna's background of dishonesty and thievery, but Betteredge stands up for Rosanna and urges Franklin not to dishonor her name now that she is dead.
This initial willingness on Franklin's part—to think ill of Rosanna—is in agreement with his character, which has been shown as questionable throughout the novel. Franklin's callousness toward Rosanna was evident in Betteredge's narrative and remains evident as he reads her letter. He is incapable, at first, of comprehending her love for him, and Betteredge has to warn him of his prejudice: "It's natural, sir, in you. And, God help us all! it's no less natural in her." Similarly, Franklin treats Lucy unthinkingly and rudely. The circumstances suggest that class privilege is at the root of Franklin's mistreatment of her. When Lucy says, "When you see a poor girl in service, do you feel no remorse?" Franklin answers, "Certainly not. Why should I?" These moments of ill judgment stand out because Franklin's narrative contains less character explanation than the other narratives. Betteredge, Miss Clack, and Mr. Bruff all included passages of explanation about themselves and all managed to reveal their personalities in their opinions and reporting of events. Franklin's narrative, however, contains little information about his background (beyond what we already knew) and reports events as a nearly third- person narrator—with little color or opinion.
The beginning of Franklin's narrative provides a return, in many ways, to the first section of the novel. The action moves back to Yorkshire, out of London, and old settings and characters reappear. The reintroduction of the Shivering Sands as a setting reminds us that Collins can rely heavily on place to set a mood when he wants. The Shivering Sands remain a site of duplicity. The Sands are both natural and unnatural, menacing to some and comforting to others (such as Rosanna), a place for hiding and a place of revelation. Franklin's perception of them under morning sunlight reflects this duplicity: "the bared wet surface of the quicksand itself, glittering with a golden brightness, hid the horror of its false brown face under a passing smile."