It requires the melting from the raw material working with oxyhydrogen flame along with the crystallisation in the melted droplets to form a cylinder. Graded nine within the Mohs-hardness scale (graded 1-10), the ensuing sapphire crystals are pretty much scratch-evidence and really tricky, ensuring an unhindered eyesight at all times. https://www.watchesworld.com/top-10-dive-watches/