It consists of the melting in the raw compound using oxyhydrogen flame and the crystallisation from the melted droplets to form a cylinder. Graded 9 on the Mohs-hardness scale (graded one-10), the resulting sapphire crystals are just about scratch-evidence and very hard, making certain an unhindered vision all the time. https://www.watchesworld.com/top-10-dive-watches/