Probably an old shell button; I'd have to tap it against my teeth to be sure- not worth anything in itself, but it's cool. If you picked up a hundred, or so, in near mint condition, you'd maybe get 20 bucks for the lot. Just sayin'. Oh, LOOK, there's one on the ground too! Woohoo, start digging! If you come across any old black glass buttons, then we're really talkin'!