Why is the Speedmaster Professional “Moon Watch” fitted with a hesalite crystal?

The Speedmaster Professional is worn by astronauts in outer space. When NASA chose this watch, they requested a hesalite crystal, which “unlike a sapphire crystal” does not break into tiny fragments on impact.This feature is very important for astronauts, as the tiny fragments of a broken sapphire crystal would pose a danger in a zero-gravity environment.