Abstract: A counter is provided for measuring counting transitions of an oscillator having an inductor in a frequency determining circuit. The inductor is at least in part air-cored and has a port therein for admitting a sample diamond for testing. First, the oscillations are up counted over a precise time interval with no sample present, and then a sample diamond is placed into the pont to change the frequency of the circuit by an amount related to the quantity of ferromagnetic material in the sample diamond. Thereafter, a down count is performed for the oscillations with the sample diamond in place over a second time interval equal to the first interval. The difference in the up counts and the down counts is determined and displayed in a digital readout as a measure of the degree of likelihood of the sample being synthetic.