It's gasoline, which is basically just dead dinosaurs.
A bit oversimplified, you should read the Steve Jobs biography, specifically the section on Gorilla Glass and what went in to getting it ready for the first iPhone.
The iPhone screen assembly has been a one piece unit for at least 4 generations. I've been in several situations where loose sand has been blowing and never had it scratch my screen. I've also had a number of windows that have never been scratched by blowing sand, lots of it. And I already mentioned getting it looked at by Apple.