The metric system is the most commonly used measurement base for diamonds (and other gemstones). The millimeter (mm) is the increment that the trade uses in measuring lengths, widths and depths. There are 25.4 millimeters in an inch and one millimeter is equivalent to .03937 inch. (1/8 inch is a bit over 3 mm.) This will give you a general idea of the relationship between an inch, which you are very familiar with, and a millimeter, which you must become very familiar with.