It involves the melting of the raw substance employing oxyhydrogen flame along with the crystallisation on the melted droplets to type a cylinder. Graded 9 to the Mohs-hardness scale (graded one-ten), the ensuing sapphire crystals are pretty much scratch-evidence and really tough, making sure an unhindered vision at all times. https://www.watchesworld.com/top-10-dive-watches/