It looks like a piece of dark tinted glass to me with the fixed window in front of it slightly lighter.
It produces a back to front tint gradient that is not unattractive.
I've seen a bunch of high res pics now and this is a good looking car.
Now it's a question of whether they price it appropriately. GM is very good at not doing that.
The other question is... how big is this car?
The CT4 has been confirmed to slot beneath it.