In 'Titanic', what is the name of the necklace's giant blue diamond?

Who plays Trinity in 'The Matrix'? In 'Gladiator', what is the name of Russell Crowe's character? In 'Gladiator', who plays the emperor Commodus? In 'Braveheart', which Scottish hero does Mel Gibson play? In 'Jaws', what is the name of the police chief? In 'Jaws', who plays shark hunter Quint? In 'Close Encounters of the Third Kind', at which landmark do the aliens land? In 'Thelma & Louise', which actresses play the title characters?