It involves the melting with the Uncooked compound working with oxyhydrogen flame and the crystallisation on the melted droplets to sort a cylinder. Graded nine on the Mohs-hardness scale (graded one-10), the ensuing sapphire crystals are practically scratch-evidence and very hard, ensuring an unhindered vision continually. Practical cookies Useful cookies https://www.watchesworld.com/top-10-dive-watches/