a precious metal testing apparatus determines the percentage content of precious metal in a specimen being tested by detecting the change in the rate of current flow through a resistive layer formed on the specimen by an application of a corrosive electrolyte on the specimen . the electrical circuit forms a fixed resistance circuit and a variable resistance circuit due to the formation of a growing resistive layer on the specimen such that the flow of current through the variable resistance circuit decreases as the flow of current through the fixed resistance circuit increase . the detection of the rate of change in current flow as a result of the increasing growth of the resistive layer can be compared to a look - up table that provides a corresponding percentage of precious metal in the specimen . calibration of the testing apparatus can be accomplished with a specimen of known purity of the precious metal .