Blue Nile says they don't use
conflict diamonds so that's good.
If I had to do it again though I'd probably try to do something different with the money than buying a diamond to meet social expectations for an engagement, or some combination like getting a Moissanite stone.
But then again I'm a lot older now and my values have changed.
But an engagement probably isn't the place to make a stand about social values on big diamonds, unless you really know her well and know she'd have a big positive reaction to that.