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Lady Verinder immediately apologizes for having blamed him for Rosanna's death.
Cuff then tells Lady Verinder that he does suspect that Rosanna knew something about the disappearance of the diamond, and that he knows of someone who can confirm or deny this.
Lady Verinder asks who he means, and he says that he means Rachel.
Lady Verinder tells him that she knows that it's absolutely impossible – it's completely against Rachel's character. But she tells him to go on, anyway.
Cuff explains: in his experience, it's not unheard of for young ladies of rich families to accumulate private debts – either for clothes, jewels, or even for gambling – that they don't want their families to know about.
If that's the case with Rachel, perhaps she wanted to pawn the diamond to pay for some debts her mother doesn't know about.
After all, he concludes, Rachel has refused to cooperate with the investigation.
Why would she do that, unless she was trying to cover up?
And she was using Rosanna Spearman as an accomplice. After all, as a former thief, Rosanna would be familiar with all the dodgy pawn brokers in London who might be willing to pawn such an expensive jewel as the Moonstone.
Cuff has two alternatives: one is to spy on Rachel and wait for her to try to pawn it.
Lady Verinder refuses, of course, to allow her daughter to be spied on.
The second alternative is to surprise her with the news of Rosanna's death and see if the shock of it might make her confess.
Lady Verinder agrees to this, but says that she'll tell Rachel herself.