It requires the melting from the raw material employing oxyhydrogen flame plus the crystallisation in the melted droplets to sort a cylinder. Graded 9 around the Mohs-hardness scale (graded 1-ten), the ensuing sapphire crystals are practically scratch-proof and really tough, ensuring an unhindered vision all of the time. Later on https://www.watchesworld.com/top-10-dive-watches/