The IDC-Rules, or in full International Rules for Grading Polished Diamonds, were drawn up by the International Diamond Council and were accepted by both the worldwide professional diamond organizations, the World Federation of Diamond Bourses (WFDB)and the International Diamond Manufacturers Association (IDMA).

In accordance with the IDC-Rules, the HRD certificate contains a proportions grade for round brilliants. Like all IDC-Rules, the procedure for proportions grading was based on the one hand on the opinion of a large number of experienced diamond professionals and on the other hand on scientific grounds.

Now, 15 years later, the IDC has decided to make a few small adaptations in the matter of proportions-grading, based on the joint experiences of the lab's working with the IDC-Rules and on the comments made by professional circles. The IDC's aim in introducing these small changes is threefold:

In short the alterations will entrail the following :

  1. The "very good" area will be slightly enlarged to include stones with a smaller table and higher crownside, so that the Tolkowsky-model (table 53%, crown height 18%) is 'now just within the limits.
  2. The evaluation of the girdle is simplified, in the sense that the nature of the girdle and the weight of the stone are no longer considered.
  3. In addition to the already current factors, the "total depth" of a stone is now also taken into account in awarding a proportions grade, which compensates for the looser approach as regards the girdle.:
  4. The occurrence or non-occurrence of certain optical effects, which in the eyes of the professionals lessen the beauty and value of the stone because they have a negative influence on the "life", is considered to be decisive for the proportions grade. These optical effects include, amongst others, the black table reflection, culet visible in bezes, fish-eye, single cut-effect. window (light leakage), deceptive weight.

In the table annexed, "IDC Proportions Grading Table for Round Brilliants", the new approach to proportions grading is illustrated.

Proportions for diamond brilliant in percentages of girdle diameter according to the IDC :

Criteria Unusual Good Very good Good Unusual
Crown angle up to 26.90 27.00 to 30.60 30,70 to 37,70 37.80 to 40.60 40,70 and up
Pavilion angle up to 38.4° 38.5° to 39.5° 39.6° to 42.2° 42.3° to 43.1° 43.2° and up
Table width 71 and up 70 to 67 66 to 53 52 to 51 50 and down
Crown height up to 8.5 9 to 10.5 11 to 16 .16.5to 18 18.5 and up
Girdle thickness extremely thin very thin thin & medium thick & very-thick extremely thick
Pavilion depth (for pointed culet) up to 39.5 40 to 41 41.5 to 45 45.5 to 46.5 47 and up
Culet size pointed to 1.9% 2% to 3.9% 4% and up
Total depth up to 52.9 53.0 to 55.4 55.5 to 63.9 64.0 to 66.9 67.0 and up

According to the IDC Rules external features of a cut diamond from a certain size up - except for loupe-clean diamonds - are classified in the clarify-grade. This means, when transferred to practical cut-grading, that a loupe-clean brilliant with easily visible damage remains loupe-clean, but that a brilliant of clarity-grade VVS 1 may, on account of external features, be graded VVS 2 or VS 1.
This method of regarding external features as dependent on clarity-grade is obviously an illogical compromise.

The table reproduced below shows the consideration of external features corresponding to the clarity-grade:

Type of External features Influence of external features on clarity-grading of diamonds with:
Polishing marks and damage Clarity-grade Clarity-grade
Loupe-clean VVS 1 and lower
Naturals and extra facets
Not visible or visible only with difficulty1) No mention No influence
Small and located on the pavilion
Not too difficult to see Mention under Remarks: "External features unimportant" No influence
Not visible when looking through the crown
Easily visible Mention under Remarks: "External features" WS 1 becomes WS 2 no influence. on lower grades
Visible when looking through the crown
Very easily visible Clearly visible when looking through the crown Description of the external features under Remarks The clarity-grade cannot be higher than VS

1) by 10X magnification
IDC Scan D.N.