4.2 Proportions
4.2.1. DETERMINATION OF PROPORTIONS
For a diamond to show an optimal combination of brilliancy and fire, it has to be polished with due attention to the angles of inclination and proportional relations between the various parts of the stone. If the angles and proportions are not optimal, this can lead to the appearance of one or more specific effects in the stone, which are detrimental to its beauty. When grading the proportions of a polished diamond, the main issue is therefore to evaluate if, and if so to what extent, these effects occur.
The most important effects that can be perceived when observing the stone perpendicular to the table are:
fish eye: the reflection of the girdle is visible through the table
black table reflection: the reflection of the table is so large that the table appears to be almost (completely) black
culet visible in bezels: the stone shows an abnormal amount of scintillation, due to the culet and the surrounding facets being visible through the bezels
window: the light falls through the stone and leaves it at the bottom without any reflection
single cut: the diamond looks as if it has less facets than it actually has,
because the difference in the angle of inclination between the facets is too small (no sharp edges)
The list of effects above is not exhaustive; there are other proportions-related phenomena which are considered to be undesirable, for instance the diameter being too small in proportion to the total depth, making the stone appear smaller than its actual weight.
4.2.2. BASIC PARAMETERS OF THE CUT
The basic parameters that can characterize the cut are for the brilliant-shape:
• the crown angle ()
• the pavilion angle ()
• the proportion of the crown height to the diameter of the brilliant-shape (% hc)
• the proportion of the pavilion depth to the diameter of the brilliant-shape (% hp]
• the proportion of the table width to the diameter of the brilliant-shape (%t)
• the proportion of the total depth to the diameter of the brilliant shape (%td)
• the girdle thickness
• the culet size
For the other shapes, basically the same parameters are involved, but then compared to the width of the diamond.
4.2.3. GRADING OF PROPORTIONS
For the brilliant-shapes, next to the actual values an appreciation can be given. The denominations are:
very good - good - unusual (medium to poor)
The measurements of the different parts of the stone can be a useful aid in determining the proportions grade, since there exists an obvious relation between these data and the appearance of the visual effects discussed in 4.2.1.
|
Criteria
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unusual
|
good
|
very good
|
good
|
unusual
|
|
crown angle (b) pavilion angle (α)
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up to 26.9° up to 38.4°
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27.0° to 30.6° 38.5° to 39.5°
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30.7° to 37.7° 39.6° to 42.2°
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37,8° to 40.6° 42,3° to 43.1°
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40.7° and up 43.2° and up
|
|
table width
(% qt)
|
71 and up
|
70 to 67
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66 to 53
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52 to 51
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50 and down
|
|
crown height
(% hc)
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up to 8.5
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9.0 to 10.5
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11.0 to 16.0
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16.5 to 18.0
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18.5 and up
|
|
girdle thickness (%)
|
0.0 to 0.5
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1.0 to 1.5
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2.0 to 4.5
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5.0 to 7.5
|
8.0 and up
|
|
pavilion depth
(% hp)
(for pointed culet)
|
up to 39.5
|
40.0 to 41.0
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41.5 to 45.0
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45.5 to 46.5
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47.0 and up
|
|
culet size (%)
|
—
|
—
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pointed to 1.9
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2.0 to 3.9
|
4.0 and up
|
|
total depth (% td)
|
up to 52.9
|
53.0 to 55.4
|
55.5 to 63.9
|
64.0 to 66.9
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67.0 and up
|
If the readings for a stone are situated in different categories, the lowest grade is considered to be the overall reading. In addition to the proportional measurements mentioned above, there are others which can have a negative influence on the final grade if they are not within certain limits, such as for example the height of the halves on the crown and on the pavilion.
4.2.4. GIRDLE THICKNESS
The girdle thickness is described in the following terms:
extremely thin - very thin - thin - medium - thick - very thick - extremely thick
The nature of the girdle can be described in the following terms:
faceted - polished – bruted
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Description
|
thickness in %
|
|
extremely thick
|
8 and up
|
|
very thick
|
6.5 - 7.5
|
|
thick
|
5 - 6
|
|
medium (slightly thick)
|
3 - 4.5
|
|
thin
|
2- 2.5
|
|
very thin
|
1-1.5
|
 extremely thin |
0-0.5
|
