It is now fairly clear that Apple is investing heavily in the production and machining of sapphire glass for its future products. Sapphire glass isn’t a new material, but it’s only in recent years, due to improved manufacturing methods, that we’ve started to see it used commercially. The iPhone 5’s camera was protected by a small piece of sapphire glass, and the iPhone 5S’s new home button/fingerprint scanner is made from sapphire glass. Historically, sapphire has been used for the front cover of high-end wristwatches. Now, it seems Apple is preparing to launch an iPhone or iWatch with its entire front face protected by sapphire glass. Why is sapphire glass suddenly so hot? After leading the pack for a few years, has Corning’s Gorilla Glass finally met its match?

What is sapphire glass?

Despite its name, sapphire glass isn’t actually a glass — it’s a single crystal of transparent sapphire. This sapphire is created synthetically, using a similar process to how the semiconductor industry grows single crystals of silicon (the Czochralski process). Basically, a tiny piece of sapphire (a seed crystal) is dipped into a vat of molten alumina (Al2O3), and then slowly drawn upwards, forming very long, carrot-shaped crystals called boules (part of a boule is pictured above). These crystals are then sliced with a saw, and shaped/polished accordingly. (The same method is used to slice silicon wafers from large silicon boules.)

This process isn’t cheap, is energy intensive, and there is usually quite a lot of wastage when it comes to slicing up the boules. It is mostly for cost reasons that sapphire glass has historically only been used in small quantities, and usually in high-ticket items (wristwatches are the obvious example). It’s also used in military settings, for shatter-proof windows and the like.

Sapphire glass, except for its cost, has some incredibly desirable qualities. It’s highly transparent between 150nm (ultraviolet) and 5500nm (far-infrared), much stronger than normal glass, and it’s one of the most scratch-resistant materials in the world — it scores 9 on the Mohs scale, one down from diamond’s 10, and quite a lot tougher than Gorilla Glass’s ~7. Its transparency, plus its very high melting temperature (~2,000C) and high thermal conductivity, make it almost uniquely suited for use in arc lamps, laser tubes, and other extreme use-cases. Oh, it’s highly resistant to acids and other caustic substances, too.

In short, it’s an amazing material that’s almost perfectly suited to the front face of the iPhone 6 or iWatch — as long as it can be produced economically. Presumably, with Apple’s recent investment in GT Advanced Technologies, and plans to begin sapphire glass production at a plant in Arizona, the folks in Cupertino believe it’s possible.

What about Corning’s Gorilla Glass?

The current king of smartphone glass is Corning’s Gorilla Glass, which is basically toughened aluminosilicate glass. Gorilla Glass isn’t actually that strong in absolute terms, but considering its low cost, weight, and thinness, it is superb for mobile form factors. In the video above, you can see the relative toughness/scratch resistance of sapphire vs. Gorilla Glass.

There are two factors here that make Gorilla Glass more desirable than sapphire: cost, and potentially thickness. Back in March 2013, when we first covered GT Advanced Technologies, one analyst said that a smartphone-sized piece of sapphire glass would cost $30. By comparison, the same piece of Gorilla Glass costs $3. In the video above, GT says sapphire glass is around 3-4x more expensive than Gorilla Glass. Corning itself says that sapphire glass isn’t thin enough — though, again in our March 2013 story, it sounded like GT had acquired some technology that would allow for the creation of thin sheets of sapphire glass.

The cost has almost certainly come down over the last 12 months, though who knows how low it has to go before Apple (and other smartphone makers) consider it viable for mass production. It would fit Apple’s M.O. perfectly if it was the first hardware company to bring sapphire glass to the consumer market. There have been numerous cases over the years of Apple investing heavily in expensive equipment, so that it can produce designs that other companies quite literally cannot copy. (I once read that Apple had a monopoly on the laser drills required to produce that glowing white light on the front edge of the unibody MacBook Pro.)

Will the iPhone be the first drop-proof and scratch-proof flagship smartphone?

There would be huge boost to consumer mindshare if Apple sold the first drop-proof, non-ugly/ruggedized smartphone — especially if you consider the bitter taste that still lingers on in the mouths of iPhone 4 and iPhone 4S owners, with their slippery, highly-non-shatter-resistant glass backs. Just imagine Tim Cook’s on-stage demonstration, as he jumps up and down on the iPhone 6, or tries (and fails) to score the screen with a piece of rock. Imagine the side-by-side drop tests on YouTube. If the sapphire screen is thick enough, it’s possible that the iPhone 6 won’t even blend.

The big question, in my eyes, is whether Apple intends to create sapphire glass covers for the iWatch, iPhone, or both. The iWatch, which will have less competition, higher profit margins, and a smaller screen, would certainly be a safer prospect for Apple. It would also be a nice differentiator against the Samsung and Sony competition, especially if the iWatch is marketed as the cornerstone of an active, “quantified life” lifestyle. It would be a huge deal if Apple led with a sapphire glass iPhone, but I wouldn’t be surprised if we have to wait another year or two until it’s economically viable.