It was worn by Louis XVI. And they called it Le Coeur de la Mer. The Heart of the Ocean.
11 seconds sound clip from the Titanic (1997) movie soundboard.
You can hear this line at 00:46:10 in the Blu-ray version of the movie.
Quote context
[...]
- I know you've been melancholy. I don't pretend to know why.
- I intended to save this until the engagement gala next week but, I thought tonight.
- Perhaps as a reminder of my feelings for you.
- Diamond. Yes. 56 carats to be exact.
- It was worn by Louis XVI. And they called it Le Coeur de la Mer. The Heart of the Ocean.
- Open your heart to me, Rose.
[...]