It's a crystalline structure, you could have dropped the watch directly on the tiles and not a jot of damage. However, if a corner of tile hits the sweet spot on the crystal then it's all over red rover.
A bit like an old world diamond cutter finding the sweet spot. A single light tap with the cleaver and you have two perfect rough diamonds. Pick the wrong spot and you end up with some polishing grit.
I'm sure it'll be back to normal after the service.
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