Tenths and hundredths of a millimeter will also be commonplace, especially when reading lab reports. 0.10 mm is a tenth and 0.01 mm is a hundredth.
If a stone measures 6.37 mm, this means it is 37 hundredths of a millimeter over 6.0 mm. If it measures 6.07, this means the stone is 7 hundredths of a millimeter over six millimeters. A common mistake misunderstands 6.05 mm for 6.5 mm, which makes a world of difference in an overall carat weight estimation and therefore in the stone’s value.