Any simple short range metal detector?
Use the simplest LC generator, where the inductor is implemented as a searching coil. 

Use frequency in the range of 100..200kHz 

Measure the frequency, using the MCU. If the metal is ferromagnetic (iron, nickel, etc) it will cause decreasing of the generated frequency. If the metal is not ferromagnetic (copper, aluminum, gold, etc.) it will cause increasing of the frequency.

Notice, that some temperature drift of the frequency is unavoidable, so provide the proper (automatic) zeroing procedure by the software.