In "Mara Clara", what does the diary really contain?

Do you remember the soap? I loved it! But I actually forgot what the diary really contains that it can expose the secrets of the past? Whose diary was it? Was it Almira's? I mean, if it Almira's, was it really stated there that Mara is her real daughter? How could it be because she really had no idea of the "switching"? Or was it Amante's diary?