Apparently it's Gorilla Glass 3 - but, it's a WELL kept secret. Almost nowhere is it mentioned it the specs. I've only seen it mentioned in a few (very few) reviews.
You'd think that Samsung and Corning would be anxious to tout that fact - which leads me to wonder if Samsung has used something else, as Gorilla Glass has had some bad press lately about it being softer and more easily scratched. Check out the video
here which explains the difference between Saphire (on the iPhone 6) and Gorilla Glass.