It consists of the melting in the raw substance using oxyhydrogen flame plus the crystallisation on the melted droplets to form a cylinder. Graded 9 on the Mohs-hardness scale (graded 1-10), the resulting sapphire crystals are just about scratch-evidence and very tricky, making certain an unhindered vision always. Later illustrations https://www.watchesworld.com/top-10-dive-watches/