It involves the melting with the raw compound making use of oxyhydrogen flame as well as crystallisation of the melted droplets to form a cylinder. Graded 9 about the Mohs-hardness scale (graded one-10), the resulting sapphire crystals are nearly scratch-proof and very tough, ensuring an unhindered eyesight all the time. https://www.watchesworld.com/top-10-dive-watches/