A rough diamond looks like any other rock until it is “cut” into many facets at optimum angles to produce maximum brilliance (sparkle) and fire (reflected colors). The quality of the cut is known as the “make”.

In 1919 Marcel Tolkowsky developed the formula for the round brilliant cutwhich has become the standard or “ideal” cut for diamonds. Since then there have been various modifications and disagreement over what constitutes the true “ideal” cut. However, the Gemological Institute of America (GIA) has developed guidelines for grading the make of round brilliant cut diamonds only.

The first two factors are the most important, and only round brilliant cut diamonds are rated by them as Class 1. The term, “ideal cut”, is no longer used by the GIA, but is still used by most of the diamond industry.

Round Brilliant Cut Diamonds only

KEY FACTORS Class 1
“Ideal” Cut
Class 2 Well Cut Class 3 Cut Average Class 4 Cut Below Avrg
Table Size
53-60% of diameter
61-64%
65-70%
over 70%
Depth
58-63% of diameter
57-64%
56-66%
under 56% over 66%
Crown Angles
34-35 degrees
32-34 degrees.
30-32 degrees
less than 30 degrees
Girdle Thickness
medium – slightly thick
thin-thick
thin or very thick
knife edge or very thick+
Polish & Symmetry
very good – excellent
good
fair
poor

More detailed charts were developed by David Atlas for fancy cut diamonds and are used by Accredited Gem Appraisers, D. Atlas & Company, Inc. and the National Association of Jewelry Appraisers. Simplified versions for these charts are shown below. (Rev: 3/02)

Princess Cut Diamonds

KEY FACTORS Class 1 Cut Premium Class 2 Cut Fine Trade Class 3 Cut Average Class 4 Cut Below Avrg
Table Size
60%-71% of diameter
58%-77%
53%-85%
Below 53%
Above 85%
Depth
64%-75% of diameter
58%-80%
56%-84%
Below 56% Above 84%
Crown Height %
8.5%-11%
6.0%-13%
4.0%-17.5%
Below 4.0% Above 17.5%
Girdle Thickness
thin-med or med.-thick
thin-thick
very thin – very thick
extra thin – extra thick
Polish & Symmetry
good – excellent
fair -good
fair-good
poor to fair

Pear, Marquise, Heart, and Oval Cut Diamonds

KEY FACTORS Class 1.
Premium Cut
Class 2. Fine Trade Cut Class 3. Average Cut Class 4. Below Average
Table Size
54%-61.5% of diameter
52%-64%
50%-70%
Below 50% Above 70%
Depth
59%-63% of diameter
58%-65.4%
46%-71%
Below 46% Above 71%
Crown Height %
12.5%-15%
11.1%-16.5%
8.7%-18.4%
Below 8.7% Above 18.4%
Girdle Thickness
thin-med. or med.-thick
thin-thick
very thin to very thick
extra thin to extra thick
Polish & Symmetry
good to excellent
good to excellent
good to fair
fair to poor
Length: Width Ratios
Class 1 & 2Pr. 1.5-1.75 M.1.75-2.25
Class 1 & 2Ht..98-1.02 O.1.33-1.66
Class 3 & 4Pr.1.26-1.99 Mr.1.51-2.49
Class 3 & 4Ht.0.91-1.24 Ov.1.26-1.74

Emerald and Radiant Cut Diamonds

KEY FACTORS Class 1. Premium Cut Class 2. Fine Trade Cut Class 3. Average Cut Class 4. Below Average
Table Size
60%-64.1% of diameter
58%-69%
53%-76%
Below 53%
Above 76%
Depth
60%-65% of diameter
58%-69%
56%-78%
Below 56% Above 78%
Crown Height %
11.9%-15%
11.1-16.2%
8.5%-18.5%
Below 8.5% Above 18.5%
Girdle Thickness
thin-med or med.-thick
thin-thick
very thin to very thick
extra thin to extra thick
Polish & Symmetry
good – excellent
fair -good
fair-good
poor to fair
Length-Width Ratios
Emerald Cut: 1.26-1.99

There are other grading systems such as “A, B and C” and a large variety of opinions that align loosely with the above charts. We have used diamond appraisers manuals to prepare these charts, so consider them relatively accurate. (Go next to Diamond Caret)