Not sure if Sapphire crystal, not glass, is a better face material than Gorilla Glass. GG has been refined over the years to improve its strength and longevity while in use as a face material. It is made in large qualities, the basic processes not not super complex, but I expect the details of applying the surface compression layer may be a bit detail oriented. Treating glass surfaces to be in compression is a very old technique, First seen in tempered windshields for cars and later in chemically strengthened glasses. The drawback to the technique is that once the surface compression layer has been breached by a scratch the inner zone which is in tension fails rapidly. Once saw a demonstration of chemically strengthened glass-ceramic, broke in tension at over 400,000 psi (2.7 GPa), or as strong as good unidirectional carbon fiber composite. The failure was spectacular, just a cloud of dust floating in the room which was preceded by a very loud bang. The GG is fairly hard, most glasses are harder than typical stuff like car keys, you might find in a pocket, so is much more resistant to scratches then the plastic predecessors and strong enough for routine use as a screen material.
Single crystal sapphire has been grown in large boules for many decades, but this is done at high temperatures and once formed the material has to be sliced and polished into facesheets. A much more intensive process then forming thin glass sheets from glass and surface treating it. I have no details on the process that GT is using for Apple to form the boules, but there are methods to make things cheaper and more efficient. There may also be improvements for the slicing and polishing methods, in the end I expect the sapphire sheets to be much more expensive than the GG, but if the absolute difference is on the order of $10 per phone, that is of not much importance to an iPhone. If the difference is $100 more that would be tougher to justify. The transmission losses through a thin layer of sapphire is nothing, but antireflection coatings have a much longer history on glass and may require significant development, but they may already have some from work on the camera covers. The Sapphire has larger fracture toughness than GG, but that only matters once the GG surface layer is breached. GG will have high strength, sapphire higher hardness. Only three common materials are going to scratch a sapphire surface, sandpaper, either alumina (poly crystalline sapphire) or carborundum (SiC) based, and sapphires, rubies and diamonds, none of which are likely to be in your pocket. Since my screen has collected zero scratches in the 2 years I have had my phone, going sapphire would not be a big improvement for me.
Sapphire laminates are used in protective windows due to the high cost of full thickness sapphire windows, 1-inch or so thick. Placing a thin, 1/8-th inch thick layer of sapphire on glass can save a lot of money. Given the cost of thinning the sapphire down to even 1mm wafers I am not sure going to 0.25mm wafers than laminating to glass would be cost effective.
Edit to add, if the particle accelerator slicer (previous post) works on sapphire like silicon, perhaps GT can make thin slices cheap enough to laminate.
Edited by wally626 - 3/5/14 at 10:18am