It includes the melting in the raw material working with oxyhydrogen flame as well as the crystallisation on the melted droplets to sort a cylinder. Graded 9 on the Mohs-hardness scale (graded one-10), the ensuing sapphire crystals are nearly scratch-proof and very hard, making sure an unhindered vision continually. Specifically https://www.watchesworld.com/top-10-dive-watches/