It includes the melting of the raw compound using oxyhydrogen flame and the crystallisation of the melted droplets to form a cylinder. Graded nine on the Mohs-hardness scale (graded 1-ten), the ensuing sapphire crystals are just about scratch-proof and really hard, making certain an unhindered eyesight all of the time. https://www.watchesworld.com/top-10-dive-watches/