That's an entirely new ring on her left ring finger. Not the giant diamond she has been wearing, which may or may not be the same one Hemsworth gave her when they got engaged in 2012. The glittery band raises so many new questions: Is this one the marker of a brand-new engagement? Why is she wearing a new diamond ring in the pool? If these two are trying to take things slowly and figure out their new relationship status, as one source told People, what's with all the ring pics?
Maybe it's just a regular old ring she bought herself, and it only fits snugly on her left ring finger, so she wore it there so as not to lose it in the water. Until we get further confirmation — or another confusing piece of symbolic jewelry takes its place — let's go with that.