The expression “to leave no stone unturned” means: To use every expedient at one’s command.
Some say that this was the reply given by the Delphic oracle when Polycrates, the Theban general, asked for aid in discovering the treasures said to have been buried by the slain leader of the defeated Persian army, Mardonius, before the battle of Plataea, 479 B.C.
Actually, according to the historian Herodotus, the answer of the oracle is usually translated, “to leave no stone unturned,” by which was meant, “to move all things.”
The English saying arose sometime in the first half of the sixteenth century and could have been common before that to indicate any exhaustive search, as for some valued object lost in the destruction of a baronial hall or the like.
The earliest mention in print is in A Manifest Detection of the Most Vyle and Detestable Use of Dice-play (c. 1550): “He wil refuse no labor nor leaue no stone vnturned, to pick vp a penny.”
Probably this was at first a variation of the older “to leave no straw unturned,” which, with the straw-littered and dust-covered floors of the Middle Ages, meant an even more exhaustive search than among stones.