Answer:
Heart of the Ocean Diamond is a mythical, fictional diamond used as a plot device in the 1997 Cameron film, Titanic.
The movie prop is probably in someone's collection -- someone associated with this film, one might guess.
When the actor (was photographed as she) dropped it off the back of the ship, she probably dropped it into a water tank on the film lot.
You can see a replica of the prop, below.