METAL DETECTOR by Brian Caswell (creator of Trilogy and Cataclysm)

In this effect you successfully dowse and identify hidden metal objects.  Borrow or supply six coins, -three similar silver coins and three similar copper coins. Borrow or supply three cups or containers large enough to cover a pair of the coins. The containers don't have to be identical. You arrange the coins in twos -  two silver, two copper and a silver and copper together. Each pair clearly has a different metallic content.

With your back turned, your spectator randomly covers the coins so you don't know which cup covers which coins. Then with your back still turned he switches the cups around, dragging the coins with the cups. Now when you turn back you obviously don't know where any particular pair of coins is.

He gives you one of the coins without letting you see anything else. You explain that metal can be used to detect other metal objects. You use the coin to 'dowse' the cups, holding it immediately above each of the cups without touching anything. You go on to determine the location of the all the coins with surprising accuracy.

A nice little mental item with the unusual theme of dowsing. There are no gaffs employed. There is no sleight of hand. All you need are the cups and the coins, which may be borrowed.