Explain how Franklin Blake is important to the crime

Franklin was appointed to deliver the diamond to Rachel for her 18th birthday. He does deliver the stone, but it disappears after Rachel leaves it in her sitting room overnite (she'd worn it to a dinner party). Rosanna, who commits suicide leave behind a note that Franklin took the stone, Rachel also says she took it with her owns eyes, but Franklin doesn't remember a thing. Thus, he becomes a suspect.