It involves the melting on the raw material using oxyhydrogen flame and also the crystallisation in the melted droplets to form a cylinder. Graded 9 around the Mohs-hardness scale (graded 1-10), the resulting sapphire crystals are almost scratch-evidence and very tough, making sure an unhindered vision all of the time. https://www.watchesworld.com/top-10-dive-watches/