This is a Magnificent Antique Large  Glass Riveted or Passementerie Button, which is a Type of Mechanical Make-up. The pieces of Glass have Smooth, Rounded Surfaces and each is Riveted onto the Pierced Metal Back, which Conforms in Shape and Pattern to the Front of the Button, This one has Hexagon Shaped Pieces around the outside and a Matte and Shiny Piece Center. There is a Large U Shaped Shank .  It is 1-9/16" in Diameter.  NBS Large