L'Affaire d'Earring

I found an earring in my front hall yesterday afternoon, specifically a diamond-tipped post for a piercing. I'm not a good judge of jewelry but it looks fairly expensive. Visitors are not unheard of here, but have not been particularly common lately -- in fact the only people I can recall in the last few weeks, who would visit wearing earrings, are my mom, yesterday's visiting nurse and several women friends who happened to help with groceries etc -- none of whom I'd like to reward with the loss of jewelry for their good deeds. I called all I could think of, and none are missing an earring. Unless it's been sitting there, unnoticed since the girl moved out upstairs (maybe a month ago now), I have no idea whose it is or how it got there. It will have to remain a mystery -- an expensive mystery, for someone. If you happen to know something, 'fess up.