If you look at a colpitts oscillator there is an inductor,split the inductor into two coils,roll the coin between the two coils,measure the change in frequency with 4097 pll,measure the damping of the oscillations with a serial adc,plot the measured results onto a two dimentional array,damping on one axis,frequency change on the other,run multiple coins through the detector you will see a gaussian distribution of both variables,you can therefore set your acceptance levels as standard deviations.Running the oscillator at 1 meg you can see the depth of the face stamping due to skin effect,and you can even see which way round the coin is.The speed of the coin through the slot must be constant,uses a photo detector to trigger the measurement cycle.Anything else i didnt say before? Whp you like me to make one for you?