I work in coin-op. A "coin mech" discriminates coins based on size, weight, ferrous content and conductivity. The latter were traditionally done simply by rolling the coin past a strong magnet. If ferrous, it would stick. More subtly, a highly conductive coin (the copper core in a quarter) would cause the coin to slow down. After it passes the magnet, it hits a knife-edge. Coins rolling a little too fast fall far and are rejected, coins rolling more slowly fall short and are accepted.
A more modern electronic mech does the same tests with a coil. It operates an ac field using the coil as the reactive component in a simple relaxation oscillator. As the coin passes into the region of changing flux, it affects the oscillation in 2 ways: frequency and amplitude. If the frequency is lowered, this is due to the coin containing ferrous content (i.e. steel) and increasing the inductance of the coil. If the coin does not greatly change the frequency, but lowers the amplitude of the oscillations, this is caused by eddy current losses in a highly conductive metal like copper, silver, gold, aluminum, etc. which reduce the "Q" of the coil.