Re: True, but...
>They are quite, but not completely, scratch-proof. However they are definitely NOT shatterproof. Any hope of a sapphire crystal surviving hammering or even a significant drop onto concrete is fantasy
It depends how it is used. A phone screen is thinner than a watch crystal, and it is only the very outer layer that you want to be hard- maybe it could be combined with a more flexible material. I'm thinking of case-hardened spanners, which are hard on the outside, more flexible on the inside so that they don't break when dropped like a drill bit will.
Many Omega watches use sapphire for the watch crystal but the models used in space used an acrylic-like material... acrylic would be less likely to shatter due to extremes in temperature, and even if it did it would be preferable to tiny shards of sapphire floating around in an enclosed environment.
Back down on earth, the things likely to scratch a sapphire watch face are harder stones in jewellery, diamond dust (if you've been a diamond blade in a disc cutter) and sometimes the anti-slip coating at the bottom of swimming pools.