myth is yakuza get beaded or pearled for the number of years the member stays in prison ... in BME (body modification ezine) there are articles where they mention in prison they use a shiv to pierce thru the skin and put a bead/pearl in. the bead/pearl comes from a can of spray paint.
there are several yakuza in the sento I go to and I haven't seen anyone beaded/pearled. it is possible incarcerated members get beaded/pearled but i just haven't met one in the flesh.