My reasoning about the ceramic iPhone rumours seems to have been correct. Instead of pricey ceramics, Apple has opted for aluminosilicate glass, which is a type of glass with a higher proportion of alumina in it. While they claim that it’s “the same type of glass used in the windshields of helicopters and high-speed trains”, it is in fact also the same type of glass as used in some other mobile devices, such as Motorola Droid. I mentioned Gorilla glass in my article and thought that Apple was already using something similar. They were not, but it appears that now they do: Gorilla glass is marketed as “environmentally friendly alkali-aluminosilicate thin sheet glass” that is “scratch-resistant, durable”, while the iPhone 4 aluminosilicate glass is “ultradurable and more scratch resistant than ever [and] recyclable”. (One of my points was that alumina or zirconia would not be as recyclable as the materials used for enclosures of other Apple products, there’s no such problem with aluminosilicate glass.)

Here’s how the material was described by Apple’s Jony Ive:

We’ve also developed a custom glass that’s comparable in strength to sapphire crystal, but about 30 times harder than plastic. This glass is not only used on the front of the phone but also on the back.

I would take it with a grain of salt: The glass may be custom designed, but that does not mean that it is substantially better than other strengthened glasses on the market. As for the strength and hardness, these two properties should not be confused: plastic materials often aren’t very hard (they are usually easy to scratch), but they may be less prone to cracking or breaking than sapphire (which is very brittle compared to metals). So while the comparison sounds favourably, it does not tell much. The glass Apple uses is certainly hard enough for everyday use and it may also be manufactured in a way that makes scratches less noticeable (like Gorilla glass is), nonetheless it will still be incomparably easier to break than a well-chosen plastic material (this is about toughness or strength, not hardness), let alone steel or aluminium.

All in all, this is clearly a more practical material than a superhard ceramic such as zirconia and it’s also a welcome update for the iPhone screen. At the same time, it may not be a wise choice for the back side.