A caliber is an instrument to measure the different dimensions of a diamond. This way we can determine if the diamond meets the needed requirements. On a caliber the measuring units are displayed both in carat and in millimeters so that we can make a clear comparison. Not only diamonds, but also rings can be measured. Here we are dealing with a ring sizer where the ring size is calibrated. Determining the exact size of the finger correctly is important to know big the ring with the diamond should be.