Johnny Ive of Apple talking about the iPhone 4 design stated that Apple developed "a custom glass that is comparable in strength to sapphire crystal but about 30 times harder than plastic."
Apple website also states on its website:
All the breakthrough technology in iPhone 4 is situated between two glossy panels of aluminosilicate glass — the same type of glass used in the windshields of helicopters and high-speed trains. Chemically strengthened to be 20 times stiffer and 30 times harder than plastic, the glass is ultradurable and more scratch resistant than ever. It’s also recyclable.
The guys at ElectronicsBreaks decided to test this by doing drop tests on a aluminosilicate glass Apple iPhone 4S and a plastic Samsung Galaxy S II with a Gorilla Glass display.
The iPhone's glass may be 20 times stiffer and 30 times harder than plastic, but it looks like it is 20x more like to crack too and 30x more like to get shattered, too.