How to Use a Millimeter Gauge

It is essential to obtain accurate measurements of your stone in order to select the right casting. This brass millimeter gauge is inexpensive and will enable you to obtain accurate measurements of the length, width and depth of your stone to within 0.25 mm. Other types of gauges are also available, such as a digital gauge.
This brass millimeter gauge is about 4 1/2 inches long. It measures in both millimeters and inches. When it comes to measuring gemstones, you want to measure in millimeters. The gauge is shown in the open position. The little round knob that you see in the center is what you use to slide it open or closed.  
To measure your stone, open the gauge wide enough to grasp the stone and then close it until the metal "jaws" are snugged up against the stone. Be sure the stone is straight in order to obtain an accurate measurement.
You read your measurement where the first line on the bottom matches up with the line on the top, and NOT where the end of the metal jaw lines up with the top. The correct measurement of this stone is 10 mm.; not 9 mm.