They can just heat the ring to evaporate the mercury (that's how they used to get the gold out of the ore, dissolve in mercury and then boil it off). Obviously these days under controlled conditions as mercury vapour isn't very nice. Alternatively, they can soak it in a nitric acid bath (mercury is soluble in nitric acid, gold not really).
Or you could go with the cool amalgam look. I wanted iridium wedding rings. Then romance butted up hard against the cold, harsh reality of economics. So we have platinum.