It involves the melting on the raw material making use of oxyhydrogen flame and also the crystallisation with the melted droplets to sort a cylinder. Graded 9 around the Mohs-hardness scale (graded 1-ten), the resulting sapphire crystals are pretty much scratch-evidence and really tough, making sure an unhindered vision constantly. https://www.watchesworld.com/top-10-dive-watches/