Rhodium plating is used on white gold only. It is necessary to achieve that amazing white finish on a white gold piece. Gold comes out of the ground in a yellow color, and the white color is created by using white alloys in the gold. It’s not possible to create the perfect white color in the gold, and therefor white gold gets plated with a thin coat of rhodium. Rhodium is a rare metal in same family as platinum.
When purchasing a white gold piece, please be aware that it will need a new thin coat of rhodium every now and then.