Kimberly Klaus

Chosen answer: It is explained in the alternate ending. It basically goes something like this: Brock Lovett and Lizzy find Rose on the stern of the ship with the diamond in her hand and asks her why she kept it all those years. Rose then says that she often thought about selling the diamond, but then it reminded her of Cal and that she could make it without his money.

Friso94

Answer: I like to think she kept it because in a strange way, although it was a gift from Cal and a reminder of his possessiveness, it was also her last physical link with Jack, the drawing having gone down with the ship, and whilst nobody but her knew about Jack she needed that reminder that he really existed and really loved her and was gunning for her in life. She likely brought it along to the wreck site opportunistically, since she knew she would never get another chance to return it to there in person. Returning it to the symbolised several things: one, that was where the constricts of her former life and of that era all died, thus freeing her - in other words, it belonged to the same world as the Titanic, and not to the modern world; two, it symbolised that she had found closure with regards to Jack's death, and that she didn't need trinkets anymore to hold onto him in her heart; and three, it was a physical symbolisation of her letting go of a huge emotional millstone that had been on her shoulders for years, as you can see from her face and demeanour immediately after having dropped it in. She couldn't have let that load go if she hadn't finally had a chance and an ear to tell her story. Probably nobody ever realised she'd been on the Titanic after the disaster, as her post-sinking name was never associated with it.