LOL, perhaps it is for "fugging cold in Denmark"...
I do wonder though who gave it to her. I sincerely doubt Mr. Fairy would give me his initial, probably mine or that of the kids...and I presume, Fred would neither. So she likely bought or chose it herself. Kind of sweet how she tries to keep the fairy tale image of her marriage alive...
I mean of all the royal couples that would wear the initials of their spouses, QEII and Philipp come to mind, perhaps Sonja and Harald, Harry wears a Millstone around his neck, but others? Perhaps Kate...and perhaps Marie, yes I can imagine Marie doing this, so perhaps that was Mary's inspiration?
I thought I read somewhere that her younger daughter’s family name is Fine, short for Josephine. So maybe she got it intending to pass it on to her when she’s old enough. Isabella (Bella) has a diamond B pendent, doesn’t she? She might have got a discount if she wore it first?