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:
- to give the evaluation a wider international character
- to simplify the grading procedure
- to define a number of visual effects more in detail
In short the alterations will entrail the following :
- 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.
- 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.
- 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.:
- 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. |