Mokume Gane
 "Mokume Gane" ist an ancient  Japanese forging technic. Several sheets of metal have to be welded before getting forged. After twisting and forging them several times, various colored patterns develop. 

The Japanese term "Mokume Gane" is an allusion on the developing grains, because in Japanese "Moku" means wood, "ME" means eye and "Gane" means metal.

A special art and concomitantly quality criterion is to forge the rings jointless,  without sample interruption.