Acrylic glass, hardened glass, and synthetic sapphire are three typical materials used for crystals in diving watches, and among them, acrylic glass is proven to be very resistant to breakage.
The typical materials used for crystals are acrylic glass, hardened glass and synthetic sapphire which all have their pros and cons. Acrylic glass is very resistant to breakage; it can easily be scratched, but small scratches can be buffed out with polishing compounds. 
