Rose's big, fat diamond necklace is definitely a big, fat symbol as well—and both the diamond and the symbol are multi-faceted. (Diamond pun!)

When Cal first gives it to Rose, it's clearly a symbol for him of their status. As he says,

CAL: It's for royalty. We are royalty, Rose.

No, Cal: what you are is an insufferable snob.

From Rose's reaction, you know she's none too pleased about getting this royal treatment. She looks kind of ill—or at least unhappy—as she reaches up to touch it. In fact, she grabs the necklace it as if it were strangling her.

For her, in that moment, it seems like the necklace is a symbol of being bought and sold—bought by Cal after being sold by her mother, that is. Seems ironic that the diamond is named "Heart of the Ocean"—isn't the ocean one of the biggest symbols of freedom, rather than of confinement?

Well, good news: Rose ends up changing the meaning of the diamond. First of all, she decides to buck convention and poses nude for Jack while wearing the rock, which immediately links the necklace with her rebellion.

Then, when she accidentally ends up with the diamond in the confusion before the ship sinks, she holds on to it—she doesn't sell it and she doesn't give it back to Cal. Toward the end of her life, when she ends up on Lovett's ship above the wreck of the Titanic, she releases the necklace back into the water.

She's giving the Heart of the Ocean back to the ocean…and she's giving her heart back to her first love, Jack.

The diamond had been a symbol of Rose's bondage with Cal at first, but by chucking it into the Atlantic—and by preventing people like Cal and Lovett from having/profiting from it—she makes it more a symbol of the free-spiritedness and love she achieved with Jack.