Mokume Gane means Burl Metal in Japanese. This is so called because the pattern looks like the burl, or patterns, in wood. Mokume Gane is very similar to pattern-welded steel, except for the fact that Mokume is used only for decorative works, not steel blades.
Mokume Gane is created with several plates of metal, two different metals. For example, it could be silver and gold, or silver and copper, etc. Normally containing silver. The plates are stacked up on top of each other, and forge welded (i.e., heated up and hammered until they are fused together), so that it turns out like this.
The billet, or ingot is then twisted, folded and formed in complex ways, and then is hammered back into the shape of a flat plate. The folding and twisting creates complex patterns in the metal, and when the metal is made into a ring, pendant, sword hilt or whatever the Mokume is to be made into, sanded and polished, the complex patterns show out, and give the pattern of wood with the strength and polish of metal.
The two metals (rarely three) must always be able to show up against each other, as with say Silver and Nickel one would have to look very close to see the separate layers. Silver and Brass, Gold and Platinum, Nickel and Copper are common sets used in Mokume Gane.
I myself have been experimenting with Mokume Gane, and am currently (don’t look Mom) working on a Silver and Copper Mokume Gane ring for my mother, who’s birthday is coming up very soon.