Visible through the caseback, the oscillating mass of the motion is delicately engraved. It's made out of 18-karat gold and decorated by hand with a barley grain sample, done by guilloche A paragon of robustness and trustworthiness, the 40 mm Oyster scenario of the new GMT-Master II is confirmed water-proof https://www.watchesworld.com/understanding-the-complex-world-of-chronometers/