One of the ways that marriages were brokered in the middle ages (especially political marriages among nobles) was for a messenger to take a portrait of the potential suitor to the other party (who was sometimes in another country). I wonder if some of the ring portraits were that kind of message (it's also possible that some of them are commemorative portraits, like a betrothal or wedding photo in modern times).
If it were a professional portrait (like the portrait of a jeweler), usually they also included the jeweler's tools, but I don't know if they always did this.
If it were a professional portrait (like the portrait of a jeweler), usually they also included the jeweler's tools, but I don't know if they always did this.