Metal Detector Tool uses the magnetic field sensor (magnetometer) of your device to detect the variation in the magnetic field that metals produce. This variation is stronger in ferromagnetic metals like iron, nickel, or steel, and they will be easily detected by Metal Detector. On the other hand, paramagnetic metals like aluminum will not be detected and diamagnetic ones like silver and gold will barely be detected. There is also a community of ghost hunters that use magnetic field detectors