After a little research, I learned she's wearing this magnificent Van Cleef & Arpels diamond zip necklace.
For those of us on a more YKK budget, it's still possible to find a necklace that looks great from behind. The key is to find a style that doesn't have a clasp so that what's usually the back of the necklace looks glamorous, and not janky, from the front.
Ok, so this Fallon Lasso Collar necklace isn't exactly sweatshirt zipper cheap, but I'm guessing it's miles less expensive than a diamond zipper.
And I know they've usually got a rep for velour tracksuits, but this Erin Fetherston (who just has a rep for being perennially cute/glam) for Juicy Couture Tassle Lariat is really cute and could be a great reverse necklace. I love the idea of lariat necklaces because they're sort of somewhere between a necklace and a Bolo tie without actually being a Bolo tie.
I'm gonna go ahead and say that these two necklaces might be a little more versatile since you can dress them down. But if you did want to dress them up to their most Van Cleef and Arpelian potential, à la Wallis Simpson in "The King's Speech," you can't do better than pairing them with a similar dress —one that's totally open from the neck to the mid-low back, and deceptively anti-décolleté in the front.
Like this one. Yowza.