A 18K karat / purity of yellow gold with 6.4mm in diameter. Calibration on a 18K sample is generally fine for testing most gold jewellery that is less than 18K. Thus, 18K Yellow Gold Calibration Disc is recommended. If you are testing gold jewellery that is 18K and above it is strongly advised to calibrate the AGT III with an 18K sample. This will optimize the AGT III’s ability to test the upper karat ranges as they are the most difficult to discern.