This had been a family object of near worthless value. It was a remnant found at the bottom of a jewellery cabinet, certainly nothing expensive or important ... just a piece of random tin that Arthur's grandmother had once owned, but hardly ever wore as it was so ugly. It had no use beyond that, but it was an ideal item to be re-purposed as part of a restrictive Marriage Bond. And after all, Ron was too self-absorbed to have parted with anything significant to bind his marriage to Hermione.