Rachel Verinder, only daughter to Lady Verinder, receives the Moonstone on her eighteenth birthday, only to have it stolen that night. Rachel is a straightforward and impassioned heroine. She is in love with Franklin Blake.
Lady Verinder is an honest and just mistress to her household in Yorkshire. She suffers from heart disease and dies a third of the way through the novel
Franklin Blake, Lady Verinder's nephew, is the editorial force behind the collected narratives of the novel—he has asked everyone to write what they know about the disappearance of the diamond, in the interests of clearing his own name. Franklin is good-natured and somewhat indecisive, perhaps because of his training in various foreign countries. Franklin is in love with Rachel Verinder.
Godfrey Ablewhite is a renowned philanthropist and leader of several Ladies' Charity Groups in London. He is a cousin of Rachel Verinder's and proposes marriage to Rachel twice. Godfrey begins to look suspicious and hypocritical during Miss Clack's narrative.
Gabriel Betteredge is the trusted house steward of Lady Verinder. He has been in service to the Lady and her family his entire life and feels a strong attachment to the family and household servants. Betteredge has a provincial, earthy sense of humor. He enjoys the novelty of both detective work and the writing of a narrative.
Ezra Jennings is the assistant to Mr. Candy. Jennings has a strange appearance—he is tall and thin, with a face wrinkled beyond his years and hair that is half white and half black—that causes him to be shunned from social interaction. Jennings is intelligent and science-minded. He seems to harbor secrets from his past and is capable of womanly depths of feeling for others.
Sergeant Cuff is a renowned detective from London. Tall and gaunt, he does not look like a member of the police force, yet his perceptive intelligence is striking. Cuff is idiosyncractic—his favorite pastime when not working is to breed roses. Cuff also exhibits a quiet sympathy for some others.
Miss Clack, niece to Lady Verinder, is an overly-pious and falsely humble Christian—a member of many of Godfrey Ablewhite's Ladies' charities. Miss Clack's main interest is in the evilness of others, whom she attempts to save with the Christian pamphlets she carries with her. Miss Clack is capable of real venom toward those she doesn't like, such as Rachel Verinder. She holds strictly traditional views on gender roles, yet would and does swoon in the presence of her "Christian Hero," Godfrey Ablewhite.
Mr. Bruff has long been the family lawyer to the Verinders—he holds Rachel Verinder, Lady Verinder, and Franklin Blake all in high esteem. Mr. Bruff has a lawyer's mind for thinking logically through the facts of the case. He is resistant to any imaginative or mystical thinking. He respects the Indians for their efficiency and tenacity.
Mr. Candy is the local doctor to Lady Verinder. He has a boyish sense of humor and a lively pride in his profession. Mr. Candy falls ill the night of Rachel's birthday from exposure to the rain and is incoherent or forgetful forever after.
Seegrave is the overconfident but not-too-meticulous local police superintendent in Frizinghall, the nearest town to Lady Verinder's estate. Seegrave's methods of detection are unperceptive and unsurprising—he suspects the servants first and bullies them into cooperation with the case.
John Herncastle, we learn in the preface, fought for the English army in India and stole the Moonstone diamond while he was there in 1799. A reclusive and dishonorable man, he eventually leaves the diamond to his niece, Rachel, in what is probably a malicious attempt to infect Lady Verinder with its ill luck.
Mr. Murthwaite is a famous traveler to India—he is fluent in languages spoken there and his thin, tanned appearance allows him to pass for a native. Murthwaite has a good understanding and a healthy respect for the Indians in pursuit of the diamond. His position is that of a metaphorical spy—entering into Indian culture and secretly reporting back to the English.
Penelope was raised with Rachel in Lady Verinder's household and now acts as Rachel's maid. Her loyalty to the Verinders, and to Rachel specifically, is a great as that of her father, Gabriel Betteredge. Penelope is clever and does not hesitate to speak her mind.
Rosanna Spearman is a housemaid to Lady Verinder. Rosanna was a thief before repenting and entering the Reformatory from which Lady Verinder hired her. Rosanna is ashamed of her past and keeps to herself in the household. She is grateful for the kindnesses of Lady Verinder, Gabriel Betteredge, and Penelope. Rosanna is a tragic figure, much like Ezra Jennings.
Lucy Yolland is a close friend of Rosanna's. She is loyal and quick to be indignant on Rosanna's behalf, even toward upper-class men such as Franklin Blake.
Septimus Luker is a seedy London moneylender.
Mrs. Merridew is Rachel's conservative aunt.
Samuel is the footman to Lady Verinder.
Godfrey Ablewhite's father is a self-made man who married into money. He is a caretaker for Rachel after her mother's death.
Mrs. Ablewhite, Godfrey Ablewhite's mother, is a lazy, but happy person. Rachel enjoys visiting with her.
Godfrey Ablewhite's two sisters are rambunctious and happy. They are referred to as "the Bouncers" by Gabriel Betteredge.
Gooseberry's real name is Octavius Guy. He is called Gooseberry because of his large, rolling eyes that protrude from his head. Gooseberry is a young boy in Mr. Bruff's employ—he keeps his eyes open for Mr. Bruff and follows people when necessary. He is sneaky, quick, and smart.