5 Reasons Why Apple Inc. Won't Put Sapphire Screens on iPads

Apple wants to see fewer of these shattered glass covers -- but at what cost?

Apple is building a sapphire screen factory in Arizona. The move could be bad news for Corning , which currently supplies Gorilla Glass for Apple's gadget lineup. If Apple moves wholesale to sapphire screens instead, it'll take the wind out of Gorilla Glass' sails very quickly.

Sapphire is the second-hardest material known to man, just behind diamond. Replacing Corning's specialized Gorilla Glass with sapphire would make the screen unscratchable -- assuming that you don't walk around with your iPhone in a pocket full of diamonds.

But scratch-proof glass isn't everything. Ask Corning's accounting officer Tony Tripeny, who spoke on the matter at a telecom conference this week. Tripeny would like to set the record straight on a few points:

• Sapphire is about 10 times more expensive than comparable Gorilla Glass slates.

• Sapphire manufacturing processes use 100 times the energy of Gorilla Glass, running at much higher melting temperatures.

• The screen formation takes 4,000 times longer.

• Sapphire is so hard that forming proper screens out of it requires very expensive machinery.

• Oh, and the extreme hardness against scratches doesn't translate into shatter-proof panels -- Tripeny said that Gorilla Glass can take 2.5 times as much pressure as a sapphire screen before cracking.

And then, once you've formed your expensive sapphire slate, this single crystal is prone to low-manufacturing yields. With glass panes, Corning and others have developed tricks of the trade to get usable end products out of slightly flawed slates. That's not possible with sapphire's single-crystal structure.

But it all comes down to cost. In consumer electronics, costs can make or break a product. So, replacing Gorilla Glass in large screens like 10-inch iPads or even 5-inch iPhones doesn't make much sense. If Apple is hell-bent on using Sapphire screens, Tripeny suggests that it has smaller form factors in mind.

Fact checking Corning's claims
All of this checks out against a sapphire-centric conversation I recently had with a materials engineer. In addition, my source says that sapphire slates have to be split just right, or the crystal structure will start playing tricks with both that supreme hardness and with the ability to let light through the screen.

So maybe we shouldn't expect sapphire screens for Apple's entire range of products. It's true that both Mr. Tripeny and my materials specialist have a bit of bias in favor of Gorilla Glass and against sapphire screens, but their talking points make sense from top to bottom.

Given these parameters, Apple might restrict sapphire screens to fairly small products, like perhaps a revamped iPod Nano -- or that elusive iWatch, where scratch resistance might matter more than shatterproofing or low costs.