I like Halle's ring but I'm not sure I feel it as an engagement ring?
If the center stone was bigger, I'd see it as an engagement ring. Rings with small colored gemstones make me think of promise or birthstone rings.
On the other hand, though, I love the thickness of the gold and the perfectly-saturated blue-green in the emerald and the fact that it's bezel-set. It's a great ring, and I'm going to look forward to seeing more yummy pics of it on her hand.