RR_NDB, read mm43's message again. He cited the ferrite and brass tests for coil tuning. Ferrite or iron will tend to lower the tuned circuit frequency. Brass brought near a coil will reduce it's inductance (acting like a shorted turn) and raise the frequency. So he got the idea that you can detect anything either magnetic or conductive. (Under salt water this may face some interesting effects when looking for aluminum rather than steel.)