Obvious throughout the caseback, the oscillating mass with the motion is delicately engraved. It's made out of eighteen-karat gold and decorated by hand having a barley grain sample, done by guilloche A paragon of robustness and reliability, the 40 mm Oyster case of the new GMT-Master II is assured water-resistant https://www.watchesworld.com/understanding-the-complex-world-of-chronometers/