There have been phone screens made of sapphire, e.g. there was a special edition sapphire version of the HTC U Ultra (https://www.theverge.com/circuitbreaker/20

There have been phone screens made of sapphire, e.g. there was a special edition sapphire version of the HTC U Ultra (https://www.theverge.com/circuitbreaker/2017/5/4/15544426/htc-sapphire-u-ultra-iphone-galaxy-scratch-resistant https://www.theverge.com/circuitbreaker/2017/5/4/15544426/ht...) which cost 150 Euro more than the glass version. I guess for just screen protection you don't need as high quality as for optical applications.