The Moonstone is an epistolary novel. This form has many advantages: the "realistic" look, the various points of view, the mingling of voices, the representation of the numerous sides of each character as appear to other characters, etc.
But there are also disadvantages. Before the documents are written, the truth has already been revealed, and all the writers have hindsight (except Rosanna). Presenting things in the past as they were, without emphasis, without bias, without foreshadowing, would be difficult, and at the same time the writers have to be, if not objective, secretive. They report the events, impressions, actions and feelings of themselves and other character in a manner that creates mystery, confusion and suspense. A professional detective/ crime writer can do so easily, which has a purpose, but it would be difficult for these characters, especially when they're involved. It's like that banal quote going around the internet "What has been seen cannot be unseen". One's remembrance of the past would have a tint of hindsight.
Or, I suppose the tint of hindsight I just talked about is that in these narratives, nobody suspects Franklin Blake (Miss Clack does, but I'm not sure if she really suspects him, or only tries to defend her dear Godfrey Ablewhite). I myself did suspect him.
If we look at the series of documents, before there is a hint of the truth, we have the narratives of Gabriel Betteredge, Miss Clack, Matthew Bruff and Franklin Blake. I reject the suggestion that Franklin could be an unreliable narrator who is actually guilty, because that means Rachel Verinder is crazy (for marrying him) and some other narrators like Ezra Jennings and Gabriel have to be liars as well, and that's too much conspiracy theory for me. What bothers me, or, what makes me think, is the part by Miss Clack. I didn't think anything while reading her narration, and I admit reading the novel very fast, a lot faster than I usually read a book and may have missed something, but it seems curious that by the time she writes her part of the story, with the help of her diary, she knows the truth and yet still presents Godfrey in that light. I say that from personal experience- once in a while I may be under an illusion about somebody, a guy perhaps, but afterwards when I realise what kind of person he is and then write about the the past, I cannot depict him as though I don't know his true character, however hard I try. By which I mean, Miss Clack's part is rather strange.
Hmmm... I'm thinking outside the book again.