I took several years of advanced silver smithing at university, and I tried and tried to master this technique using silver, copper, and bronze. It's insanely difficult, I got close but never quite got it right.
I really wanted rings like this, but my husband won't even wear a shirt in anything other than gray, blue, or brown, much less any kind of interesting jewelry! So plain bands it was. I still plan on getting some other piece of Mokume jewelry (a pendant, perhaps, or maybe a ring for my right hand) at some point.