A diamond gauge measures the width, length and depth in millimeters of a diamond or other gemstone. More advanced diamond gauges also give the user an accurate carat weight for the stone. Others require the use of a conversion method to determine carat weight.
A diamond gauge can be used on a mounted stone or a loose one. It helps determine the diamond's value. A gauge is usually required to measure a stone for appraisal or documentation purposes. It helps determine the quality of the gemstone cut and also checks the mount size and the ring size. Computerized gem gauges calculate carats by themselves without the use of conversion tables.