TTF Datejust/Day-Date Dial Window from Duke?

Hey guys. I want to purchase a Datejust/Day-Date (haven’t decided yet) from Duke and I have asked him what’s the material of the dial window (more rhetorical than literal tbh haha) and he claims it’s sapphire. Does anybody have the knowledge if thats actually true? I mean it seems to good to be true for a 145$ rep to have a sapphire glass, isn’t it?