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Ideal Cut Diamonds.  Ideal proportions, measurements for perfect diamond cutting
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Approximate Diamond Cut Grade Chart for round brilliant diamonds

If you are not sure, trust the opinion of the Expert, the independent Grading by a reputable Gemmological Laboratory

Proportions:                   Total depth %           Table %              Crown %        Crown angle °    Pavilion %         Pavilion angle °

Excellent:                          58.0 - 63.5               52.0 - 63.0         12.0 - 16.5       31.5 - 36.5        42.0 - 44.5        40.0° - 41.7°

Very Good:                        55.0 - 65.5               52.0 - 66.0         10.0 - 17.5       28.5 - 38.0        41.5 - 45.0        39.7° - 42.3°

Good:                                53.0 - 69.0               50.0 - 70.0          8.5 - 19.0        26.0 - 40.0       41.0 - 46.5        39.4° - 42.9°

Fair & poor:                       less than                  less than            less than         less than         less than           less than
                                        
Girdle
Excellent:                          Thin to slightly thick

Very Good:                        Very thin up to thick

Good:                                Extremely thin to very thick

Fair & poor:                       Other

Symmetry: is the balance and regularity / evenness of facets.
Facets are compared in opposing pairs,
defined  as following:

Very Good. (some laboratories describe this by Excellent)

Good. (some laboratories describe this by Fair)

Poor.

Unusual.

Very Good. (some laboratories describe this by Excellent) - Good. (some laboratories describe this by Fair) - Poor

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This page was last updated: February 2, 2012
Actual ideal cut diamonds - grading chart
ideal cut diamonds
100 %
Total depth 58.0 - 63.5 %
Ideal Table size 52.0 - 63.0 %
Crown depth12.0 - 16.5 %
Crown angle 31.5 - 36.5°
Crown
Pavilion  hight 42.0 - 44.5 %
Pavilion angle 40.0° - 41.7°
Cullet: pointed 0 - 1 %
Girdle thickness: 1.8 - 2.0 %
Approximate Ideal proportions for fancy shapes:

Shape           ideal lenght          width

Oval             1.30 to 1.68          1.00

Marquise    1.75 to 2.25          1.00

Princess     1.00 to 1.10          1.00

Pear             1.50 to 1.75          1.00

Heart            1.00 to 1.10         1.00

Emerald      1.50 to 1.75          1.00

The depth percentage of an fancy cut diamond is generally determined by dividing the length to width ratios
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Learn about Ideal Cut Diamonds

The ideal cut is the most important element to reflect brilliance
It is determinable by 3 separate descriptions :

The sparkling quality of the ideal cut is to be determined by 3 separate factors:
  1. Proportions
  2. Polish
  3. Symmetry of its facets

Important Notice
If you are not a professional or familiar with the cut grading, trust
the second opinion of the Expert, the independent Grading by a
reputable Gemmological Laboratory. Best view the diamond loose
and its genuine grading document at the diamantaire.

With the right proportions a brilliant round diamond will be called
'ideal cut',
not an 'official' term to describe cut quality by the way.
Ideal Cut
How Diamond Laboratories grade Cut - Polish - Symmetry
View image of certified ideal cut

1. Proportions:
Excellent

2. Polish
Excellent

3. Symmetry
Excellent

4. Hearts and Arrows
Premium Ideal Cut: Hearts & Arrows is the Art of cutting and for far the best cut!

Ideal symmetry & proportions of fancy shapes      ( Horse shaped diamond cut )

Ideal cut diamonds, Hearts and Arrows and Triple Excellent command a price premium in the world's market reflecting by far the real value -  most expensive!

Ideal Cut Diamonds explained