the apparatus comprises a parallel coil and a capacitance , the coil having an air gap in which an unknown specimen is inserted , a source of alternating voltage applied to the coil - capacitance combination , scanning means for automatically varying the value of the capacitance in steps , thus varying the phase of the voltage across the coil , means for digitizing the phase of the voltage corresponding to each capacitance value , a memory in which those digitized phase values are stored , a reference memory containing digitized phase values for known specimens , and means for comparing the digitized phase values of the unknown specimen with those of known specimens . a digital tolerance control circuit is also provided which accommodates deviations between the reference and the known phase values , up to predetermined amounts , and means are provided for displaying the generic name or grade of the stored phase values which match the phase values of the unknown specimen .