It will involve the melting of the raw material working with oxyhydrogen flame plus the crystallisation in the melted droplets to kind a cylinder. Graded 9 to the Mohs-hardness scale (graded 1-10), the ensuing sapphire crystals are just about scratch-proof and really hard, guaranteeing an unhindered eyesight all the time. https://www.watchesworld.com/top-10-dive-watches/