Dave,
I did a quick web search and came up with this very interesting FAQ page (see link below). As for your specific question, the author states the following:
"The crystal on the Speedmaster Professional is made out of Hesalite, a brand name and a sort of acrylic. Omega chose that material because of its resistance to shock and extreme temperatures. If struck hard against something Hesalite will crack BUT it will stay together as opposed to mineral/sapphire crystals that are scratch resistant but will crack and fall apart if abused hard enough. Hesalite will scratch more easily than glass BUT you can easily polish out scratches that are not too severe. My personal opinion is that a few scratches gives the watch a personality and should not be considered as imperfections. All Speedmaster Professionals and the 1957 replica have Hesalite cystals (except Moonphase)."
Just my two cents, but I have worked in the plastics industry for almost 7 years and I've worked with acrylics including the tradename Plexiglas. These acrylics are MUCH more prone to scratching than sapphire crystals, which are nearly impossible to scratch. Personally, I would never buy a high end watch without a sapphire crystal, mainly because, unlike the author, I DON'T want to see scratches on my crystal. The few advantages of Hesalite do not compensate for its poor scratch resistance when compared to a sapphire crystal.
