Sapphire coating on mineral crystals is the best compromise between scratch-shatter resistance. Hardened mineral crystals are used in most diver watches since the sapphire type is more prone to shatter into pieces under heavy underwater pressure.
On the other hand, sapphire crystals are more scratch resistant than hardened mineral ones but as stated before, a sudden hit might easily break them apart, even in "dry" situations.
The anti-reflection treatment is used for the inside surface of the crystal. Afaik, the outer anti-reflection performance can be achieved only by using dome-shaped crystals and not flat ones. I have three chronographs that use sapphire crystals with anti-reflective treatment and I can assure you that their outer surface reflects sun rays like a plain mirror... So, if you want outer a-r treatment, have a look at dome-shaped crystals (either sapphire or not).
Most extra large diver watches (down to more than 200m) are made of hardened mineral crystal not only because a large sapphire crystal costs a whole lot more than a mineral one but also because the pressure vs crystal surface renders the crystal suffering a lot while working very deep. So, a sapphire crystal living at 200m underwater will easily break even while facing a gentle hit. So, under these depths, even its scratch resistance is eliminated (or is rendered insignificant) by its low impact tolerance. Add to that the extra pressure added while moving your hand rapidly and its no brainer why a mineral crystal is the best choice for divers.
So, when both worlds meet (hardened mineral with sapphire outer coating), you've got a crystal that will survive almost every nasty situation. The thing is that not all mineral crystals have pro-level sapphire coating implementations, so some of them tend to peel-off after a couple of years...:sneaky2:
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