[jonifico]

This video explains the reasons quite well. https://www.youtube.com/watch?v=kVQbu_BsZ9o

Althought saphire is in fact stronger, it is it's lack of flexibility and absorption properties that don't make it as viable for phone use.

[nodata]

Why did they buy a plant in Arizona then?

[zackangelo]

They're using it on the watch.

>On most Apple Watch models, the display is laminated to a machined and polished single crystal of sapphire.

http://www.apple.com/watch/technology/

[paul_f]

The iPhone is a camera that makes phone call. There's a lot of camera lenses to make.