The iPhone 6 currently uses Corning’s Gorilla Glass. There was some speculation that they would start using Sapphire glass but I guess they’re not ready yet. I have had my iPad 2 for years now and the glass is still perfectly fine. My new iPhone 6 uses a more developed version of the same glass. Will the glass break? Yes. Will the glass scratch? Yes, if you carry your phone in the same pocket as your car keys, the screen will scratch. It’s not impervious, just highly resistant.
I do have a real life controlled experiment going on in the family. My wife and one son have their iPhone 6s in a case WITH a screen protector. Myself and my other son carry our iPhone 6s around without ANY protection whatsoever, no case and no screen protector. So far they all still look perfectly fine. We’ll se how this experiment turns out.