They represent something special. sentimental, similar to a teddy or something our parents gave us.
Explanation:
They reminded her of a better time and gave her hope. Whilst her diary was a record of what happened to her, like a time capsule. So, that she would be remembered forever and wouldn't just be forgotten. Whereas a knife doesn't represent a story or her life.
Douglass then asks a rhetorical question: "Are the great principles of political freedom and of natural justice, embodied in that Declaration of Independence