Whitebeard said that when "the one" Gol D. Roger has been waiting shows up, he'll find the One Piece and he will turn the whole world upside down. I can't recall any statement that declares that One Piece is some kind of a treasure like gold or something.
It is just said that it is Gol D. Roger's treasure which he left in that place. So I wonder if it has ever been stated that One Piece is something tangible or something theoretical, something virtual?