Diamonds require two measurements of the diameter of the crown, or top, while the round brilliant cut requires an additional measurement of the depth, or height, of the diamond. The two measurements, or three in the case of the round brilliant diamond, along with the carat weight, are used to identify a particular diamond. All dimensional measurements are done in millimeters.
Round cut diamonds are measured directly across the center, with the second measurement at a 90-degree angle to the first. Even the best cutters rarely produce a diamond that is truly round. They come close, as in this example measurement of 6.67 by 6.65 by 4.01 millimeters. The last figure is the height.
The near-perfect measurement seen above means this diamond has good symmetry, and the cut stone has the best light refraction, creating more "flash." To be considered a proper cut brilliant diamond, it must be within certain limits. For example, a 0.5-carat diamond must have no more than a 0.05-millimeter difference in the two crown measurements. A larger 4-carat diamond must have a variance of no more than 0.16 millimeters.
To measure a pear-shaped diamond, a length and width measurement is taken and then divided to get the length to width ratio. One example is a diamond with a length of 7.5 millimeters and a width of 5 millimeters. The 7.5 is divided by the 5 to create a ratio of 1.50. A pear diamond is considered more appealing if it has a ratio of between 1.50 and 1.75.