In the movie, The Princess Diaries, Mia receives a diary from her grandmother in which to record her new royal life. The diary had a lock for which the key to open it was a locket Mia wore around her neck.
I found a pretty good replica online. It actually opens with the locket and can be personalised or created for other fandoms. Rather pricey but it is handmade, and refillable. The site where it is sold also sells some other interesting movie related things..