I've recently become interested in some of the Seiko mid to high range divers and noticed something. The Seiko divers that use a sapphire crystal cost thousands. Even the MM300 at $2400 USD uses Hardlex. I don't see sapphire until I get up into the 600m or 1000m divers.
I have a Seiko JDM SARB021 that sports a sapphire crystal that cost $450.
What exactly is Hardlex? Does it have a benefit over Sapphire?