Crystal Ball by Richard Raven

Effect: An invisible black crystal ball is imagined. A spectator selects a card and signs the face. the invisible ball is pushed into the deck. When the cards are spread out, a black spot is found on the back of one card. The card is turned face-up revealing the spectator signature. The signed card is placed over the spectators open hands and then magically, a black ball falls from the card into the spectator's awaiting hands. Upon closer examination, the back of the spectator's card reveals the black spot is has vanished.