The Heart of the Ocean
"Louis the XVI wore a fabulous stone, that was called 'The Blue Diamond of the Crown', it disappaered in 1792, about the same time old Louis lost everything from the neck up. The theory goes that the 'Crown' diamond was chopped too, to be cut into a heart like shape that became known as the "Heart of the Ocean. Today it would be worth more than the Hope Diamond."  
-Brock Lovett
In the case of the Heart of the Ocean, this is one area where Cameran takes his greatest "dramatic libertys." History will willingly confirm the existance of the "Blue Diamond of the Crown" or "French Blue" and it was indeed owned by King Louis the XVI of France. But this is precisely where history ends, and storytelling begins. While the jewel was part of the french Royal Jewels, it was worn on a ribbon by both Louis, and his wife Queen Marie-Antoinette, who is known to have had a voracious appetite, near obsession, for diamonds. In the case of history, the diamond was at the time one hundred twelve and three/ sixteenths carats if I'm not mistaken. The diamond was at the time a triangular shape, and crudely cut.
King Louis XIV bought the diamond in 1668 and in 1673 he had it then cut by the court jeweler Sieur Pitau in 1673, resulting in a much nicer looking, but much smaller stone at sixty-seven one/eighth carats. It was now that the actual stone did indeed attain a heart shape. In the royal inventories, it's color was described as an intense steely-blue and the stone then aquired it's name as the "Blue Diamond of the Crown" or "French Blue" It was now that the diamond was set in it's gold and ribbon setting, to be worn on ceremonial occasions.
King Louis XV, in 1749, had the stone reset by court jeweler Andre Jacquemin, in a piece of ceremonial jewelry for the Golden fleece (Toison D'Or). In 1791 after an attempt by Louis XVI and Marie-Antoinette to flee France (with the diamond) the jewels of the French Royal Treasury were turned over to the government. During a week-long looting of the crown jewels in September of 1792, the diamond was stolen.   
In 1812 a deep blue diamond described by John Francillion as weighing 117 grains (4 grains is equal to one carat) was documented as being in the possession of London diamond merchent, Daniel Eliason. Strong evidence indicates the stone was in recut "French Blue" and the stone known today as the Hope Diamond.
It is at this point that reality completelly differs from Cameran's fantasy. The film states the diamond was recut at the death of Kind Louis XVI into a heart shape.
In truth, this is actually the time that the diamond lost it's heart shape. Brock Lovett says, "Today it would be worth more than the Hope Diamond." Well, being that the diamond IS the Hope Diamond, this is quite impossible. But we must of course remember the entire story aspect is fantasy, and without the diamond, the story would never have been told. The diamond is indeed the axis on which the whole story turns, the piece, without which the story would have no substance.
Rose wears the "Heart of the Ocean" necklace exactly twice in her time aboard Titanic. First she wears the neckace on the night of April 12th, 1912, the night her finiace Cal gives her the lavish, impossible sized jewel to Rose as a symbol of his love for her. The gift although says more of Cal than it does of Rose. It demonstrates the fact Cal knows nothing of emotion, but only of wealth, lavishness, and the power both provide. Cal  expresses this trait in himself later in the film when he attempts to buy his own seat in a lifeboat to save himself.
Rose wears the diamond (and nothing else) once more when she is sketched by Jack. It is this sketch, after surviving 84 years at the bottom of the ocean, which Brock Lovett discovers, and allows Rose to tell her story, by first asking Brock, "I was just wondering if you had found the Heart of the Ocean yet Mr. Lovett?"
According to the story, the massive center stone is not a saphire, but a massive, dazzling diamond. It's weight: fifty-six carats. In reality, the Peterman diamond is even larger than the film specifies, at seventy-five carats. Around this unbelieveable stone dances the fire generated from the inner reflections of over fourty small rounded diamonds.
The heart is suspended by an eighteen inch chain studded with eighty four much larger round diamonds, set in a platinum setting. The heart itself measures an impressive one and three-fourths inchs, reinforcing Rose's statement of, "It was a dreadful heavy thing..."
The best, and only authorized replica of the "Heart of the Ocean" was created by the J. Peterman Co. who sold the necklace for $199 at the time. I was lucky enough to buy one off e-bay a little over a year ago for only $125. They are still regularlly on e-bay, but they usually exceed $200 to $300 or more now. 
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