I almost thought she would have a Colombian emerald, since she wears them so much on the red carpet. I would've loved to have seen someone as high-profile as her picking a colored gemstone over a diamond in an engagement ring. But Brad Pitt helped design it, and he's a diamond guy, as we saw with the ring he gave Jennifer Aniston. Darn.
But: I love the size of the diamond. It's not in-your-face-I-have-money-and-can-afford-huge-expensive-things; it's just right for her hand. (From this angle, anyway.)
ETA: IT'S CONFIRMED! They're engaged!!