This is from the same Elie Saab collection as the green dress Nina Dobrev wore to the PCAs.
The consistency here seems to be an “almost… not quite” deployment of lace, which Fug Nation correctly noted looked like a strange skin ailment on Dobrev, and which here is stretched over her chest like a window and helps give off the impression Chastain just polished her brass knockers. The color is still great, though, and the rest of it is breezy and pretty. The sunset at Nipple Beach is really the most distracting part — well, that, and the fact that this last-ditch deployment of green has me fearing she’ll go baby pink or nude at the Oscars, which will make my eyeballs explode, and y’all KNOW how much I hate even the mere mention of eyeball-related shenanigans (and if you didn’t, you do now), and so you know I MEAN IT.
Later, Chastain pulled out this number: