No, he drops the Pumice, but you can steal a Pumice Piece.

You see, there are 2 ways to get a Pumice. Either Synthesize 2 Pumice Pieces(3 are available in the game, 2 found, 1 stolen from Ozma) or simply defeat Ozma. He always drops a Pumice.

If you think I am wrong, you obviously haven't beaten Ozma, or simply forgot.