Mokume Gane literally means "wood eye metal" in Japanese, referring to the look of knots in wood. The technique arose from the Japanese sword making tradition around 300+ years ago.
Sheets of alternating layers of different colored metals are fused together using high heat and pressure. There is no solder used in the technique. Once the layers are fused together, the patterning on the surface can begin.
Different methods of twisting, carving and grinding into the billet give different patterns. With time, the craftsman can replicate a certain patterning style, but no two pieces are ever identical.