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I have the Apple Watch series 4 with the aluminum case with the ion x glass screen and it broke so can I buy the sapphire screen instead because it is claimed stronger and only 20 differences
Well… Its not that simple! First a Sapphire display is not as easy to see as it has a bit more refraction. As far as being stronger not really! It is easier to fracture! Just less likely to get scratched. I would strongly recommend you stick with the Ion glass. When I was fixing Apple Watches (series 0, 1 & 2) I saw more broken Sapphire displays than the Ion. I had one customer who in less than two weeks broken his display again dropping in on to a concrete floor. The third display he went with the Ion glass ;-} iFixit currently does not sell the Series 4 display, you’ll need to try to locate a source for the display. You will need to transfer the NFC chip to your new display. Otherwise you’ll need to go to Apple to get your watch fixed.