1 String, 1 metal, 1 gemstone (of any size)
This creates a pendulum which functions for gems like the dowsing rod does for water. The higher the Q and cut of the gemstone, the greater the range it will work. The pendulum will also give greater accuracy and distance for gems of the same type as those used in construction of the gem, but it will work for any gemstone.
The greater the size of the gemstone, the greater its range for seeking stones. The better the cut, the more accurate (narrower slice).
Both these values are, of course, modified by the final Q of the pendulum, and the users Psyche.
Additionally, they will only have limited uses before they break. The number of uses is once again governed by the size and cut of the gem, and the final quality of the pendulum.
This idea was inspired by the complaint regarding "mine a certain gemstone" quest for the gemcutter's credo.