This is a 4mm wide silver band with a 2mm wide 9ct yellow gold sleeve.
The silver ring is flat on the outside and rounded on the inside to make it really comfortable to wear. A 2mm section of the silver is removed and a 9ct yellow gold piece is soldered on. A thin line is engraved to clearly define the two different colours.
This ring can come in different widths, in any finger size and it can also be set with diamonds. To demonstrate what is possible, there is a photograph of a 5mm version of this ring set with diamonds.