I don't really think that it's actually her's. But at the same time, they must do a DNA test on it, and we know they arrest Mary so...
I'm conflicted...

I'm sticking with the theory that the box Ruby found was what Mr. Gold was burying in 1x07. So given the timeline it shouldn't be hers.

I just can't see Kathryn's story ending so abrupt like that.

Regina ripping out hearts is kind of her thing, this heart could be anyone's and I think Kathryn is alive.

Well considering that Regina has a wall of hearts i really don't think Kathryn is dead is probably got taken by somebody

regina has a creepy closet full of hearts, it could be anyone of them if i had to guess gram's heart. he is dead and why would she keep is heart?

I'm sure Regina planted it from her wall full of hearts. It's easy enough to transfer somebody's fingerprints from one object to another, and Kathryn & Regina met occasionally, so no problem there. I'd love for them to dig up Graham for an autopsy & find his heart isn't in his chest!

I need Abigail and Frederick to get their happy ending, like everyone else will. I'm sticking to the idea that Kathryn is not, and never will be, dead.