In Titanic (1997), towards the end after the ice berg accident, Rose decides to go back and tell her mother and fiance about it to warn them. "Now that one of my missing items has returned, I know where to find the other" and at that point, Jack is searched and found to have a Diamond in his coat pocket (a coat in which he had "borrowed" earlier).
We see Rose ask Jack to put the diamond back in the safe after he is done drawing her in the nude while wearing it. It seems obvious that he has stolen it, but if that is the case, why? An easy answer is that because he is a poor boy with a 3rd class ticket, he has plenty of reason. I believe this to be too easy of an answer and don't quite think he has stolen it at all, but has been framed or simply "borrowed" the wrong jacket. But, since he acquired the jacket before he met with rose to draw her, we know the diamond was not at that point missing.
So, did Jack really steal the Heart of the Ocean diamond necklace?