It involves the melting with the raw material utilizing oxyhydrogen flame as well as crystallisation of the melted droplets to kind a cylinder. Graded nine on the Mohs-hardness scale (graded 1-10), the resulting sapphire crystals are almost scratch-proof and really hard, making sure an unhindered vision all the time. Specifically https://www.watchesworld.com/top-10-dive-watches/