I see that glass or zirconia is radio transparent, but what about the weight.
If you are looking at the density of glass it is ca. 2x that of plastic.
Worse for zirconia: zirconia has a density ca. 5x that of plastic.
The plastic parts for mobile phones are really thin, I don't think you can make a glass or a zirconia part that thin -> which ads more to the weight.
And don't get me started on the price of glass or zirconia.
I don't want to dismiss the possibility of...