This is something I used to find on the sand of the beach every day when I was a child, nowdays it rarely happens. It belongs to a shell called ASTREA RUGOSA , it actually is "the door" of the shell. It is used a lot in jewelry. Big or small,it is a lucky charm, and is beautiful. It is called Occhio di Santa Lucia, the Eye of Saint Lucia, this is probably related to the legendof the saint.