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.
>On most Apple Watch models, the display is laminated to a machined and polished single crystal of sapphire.
http://www.apple.com/watch/technology/
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.