There are very few trained craftsmen in the whole world who can work with the unique Japanese mokume-gane technique (mo = wood, kume = texture, gane = metal). What is more, the metal used for mokume-gane is not easy to work with: it is high carbon steel with bronze, brass and copper. They all have different melting and forging temperatures and that is the main challenge.
However, the craftsman that can handle this complex alloy is going to get truly unique and incredibly beautiful patterns on the metal. When both a watch case and a watch dial are made using the mokume-gane technique, the result is perfect harmony.