glass can only be scratched by something with a harder compound than it, sapphire glass is f**king hard, think it may be a diamond is the only thing that can scratch it, i would need to google it though.
a car windscreen having a stone hit it at 70mph will indeed crack, its a small hard thing hitting something at speed, just like when you drop an iPhone on the ground the glass shatters.
i dropped my iPhone on the floor, back down and the glass at the top and bottom both smashed completely, the camera lens didn't have a scratch on it, and hasn't after 2 years of ownership and placing it on objects without a case, its bloody tough, you won't be scratching it unless its intentional.
they wanted to make the front glass from the same material i believe, it didn't happen this time around, maybe in the future, i suspect its either down to cost, or manufacturing or supply difficulties, as its the biggest issue with the phone, one drop and its likely to smash.