For round brilliant cut diamonds, the measurements help you determine total symmetry. Perfectly round would mean more brilliance in a diamond. Though no diamond is quite perfectly round, you want to find one that is very close.
The first two figures in the measurements of a round are the maximum and minimum diameters. These two diameters should not vary by too much.
Example: A round diamond with measurements of 6.50 x 6.56 x 4.72. This means that the diameter varies by only 0.06 mm, which is quite acceptable for a 1-carat diamond as you will see in the following list.
Below are the opinions of Fire And Ice of acceptable variances for round brilliant diamonds in the popular sizes. Less variance is generally better.
Carat Weight / Acceptable Variation
0.50 carat / 0.05 mm
0.60 carat / 0.06 mm
0.70 carat / 0.07 mm
0.80 carat / 0.08 mm
0.90 carat / 0.09 mm
1.00 carat / 0.10 mm
2.00 carat / 0.12 mm
3.00 carat / 0.14 mm
4.00 carat / 0.16 mm
5.00 carat / 0.17 mm
The Following are Round Brillaint Cut Proportions used in Calculating a Brilliant Cut Diamond's Formula:
Weight Diameter Depth
0.03 carat 2.00 mm
0.04 carat 2.20 mm
0.05 carat 2.40 mm
0.06 carat 2.60 mm
0.07 carat 2.70 mm
0.08 carat 2.80 mm
0.09 carat 2.90 mm
0.10 carat 3.00 mm
0.11 carat 3.10 mm
0.12 carat 3.20 mm
0.14 carat 3.30 mm
0.15 carat 3.40 mm
0.16 carat 3.50 mm
0.17 carat 3.60 mm
0.20 carat 3.80 mm
0.22 carat 3.90 mm
0.23 carat 4.00 mm
0.25 carat 4.10 mm
0.30 carat 4.20 mm 2.50 mm
0.33 carat 4.40 mm 2.60 mm
0.35 carat 4.60 mm 2.80 mm
0.40 carat 4.80 mm 3.00 mm
0.45 carat 5.00 mm 3.10 mm
0.50 carat 5.20 mm 3.20 mm
0.55 carat 5.40 mm 3.30 mm
0.65 carat 5.60 mm 3.40 mm
0.70 carat 5.80 mm 3.50 mm
0.75 carat 5.90 mm 3.60 mm
0.78 carat 6.00 mm 3.70 mm
0.85 carat 6.20 mm 3.80 mm
0.90 carat 6.40 mm 3.90 mm
1.00 carat 6.50 mm 4.00 mm
1.05 carat 6.60 mm 4.10 mm
1.15 carat 6.80 mm 4.20 mm
1.25 carat 7.00 mm 4.30 mm
1.35 carat 7.20 mm 4.40 mm
1.50 carat 7.40 mm 4.50 mm
1.60 carat 7.60 mm 4.60 mm
1.75 carat 7.80 mm 4.70 mm
1.85 carat 8.00 mm 4.80 mm
2.00 carats 8.20 mm 4.90 mm
2.15 carats 8.40 mm 5.00 mm
2.25 carats 8.60 mm 5.10 mm
2.40 carats 8.80 mm 5.20 mm
2.50 carats 9.00 mm 5.30 mm
2.75 carats 9.20 mm 5.40 mm
3.00 carats 9.40 mm 5.60 mm
3.10 carats 9.60 mm 5.70 mm
3.25 carats 9.80 mm 5.80 mm
3.50 carats 10.00 mm 5.90 mm
3.75 carats 10.20 mm 6.00 mm
4.00 carats 10.40 mm 6.10 mm
4.25 carats 10.60 mm 6.20 mm
4.35 carats 10.80 mm 6.30 mm
4.50 carats 11.00 mm 6.40 mm
4.85 carats 11.20 mm 6.50 mm
5.25 carats 11.40 mm 6.60 mm
5.50 carats 11.60 mm 6.70 mm
5.75 carats 11.80 mm 6.80 mm
6.00 carats 12.00 mm 6.90 mm
Diamond proportioning began in 1919 when a college student, Tolkowsky, used numerical formulas in a thesis to achieve the modern brilliant cut. He did this by cutting a diamond so that all light entering the stone followed a controlled path to exit the top of the diamond.
In 1919 the diamond cutting industry was not advanced enough to apply Tolkowky’s mathematical angles and dimensions. Instead it used variations of the cut, based on what the technology of the time could do. As technology advanced, so did the ability to better approximate the ideal Tolkowsky cut, which provides the best return of light in a diamond. It is possible for an experienced diamond grader to estimate, within a decade, when an older diamond was cut by observing where on the technology timeline its proportioning falls and how well is achieves Tolkowsky's ideal cut formula.
The best cut casts a perfect shadow on a wall when a beam of light is shone on it. Very few will do that because, due to many factors, most diamonds deviate from the perfect cut. The perfect cut diamond is difficult to achieve, and many people differ as to what is a perfectly cut diamond.
A properly cut diamond is proportioned so that no matter where light enters the stone...from the top, sides or bottom… the light is directed out the top of the stone. This is difficult to achieve, but mathematically possible. The reason is that a diamond is cut to capture light entering it from a 360-degree sphere.
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