I don't know how to say, it is hard and it is also very easy.
As it is very hard (second to diamond ) that means it could be brittle!
I have tried to use hammer to wrack the Sapphire crystal, I was quite hard to crack it at all.
My when my daughter used the watch for three days, she brought back to me saying that it was cracked without any notice on how did it happen. I was a bit mad! How could you break it!?
What should we do if the sapphire crystal was cracked?
We should pull out the crown stopping the watch to move as if not the broken sapphire debris would damage the expensive dials by the moving hands.
Send to Rolex Service centre for repair.
It is about HKD$1,500 for a replacement on a Sapphire Crystal, friend of mine had sent his Submariner M116610LN for replacement of the cracked crystal, he had to pay for HKD$5,000, of course, Rolex will ask you to change the water sealing ring, clean it a bit. 3 weeks!
As for my daughter's watch, I sent to a good old watch smith at Sham Shui Po, he immediately replaced for me with Genuine Rolex Crystal, HKD$800 in just 35 minutes.