The cut, or proportion, of a diamond is measured in percentages relative to the diameter of its girdle.
The girdle diameter of each diamond is always considered 100%. Example: The girdle of a diamond
measures 10 millimeters (100%). The table measures 5.6 millimeters. The total depth measurement
is 6.1 millimeters. The diamond would be described as having a table of 56% and a depth of 61%.
The table and depth percentages are the key to determining the best proportions.
The ratio between the depth and the width of the diamond will determine how the light will be reflected
back to you. The most light it reflects the better the cut is rated.
According to Marcel Tolkowsky (Mathematician and gem enthusiast) who, in 1919 developed the round
brilliant cut, the ideal dimensions are as follows:
- Table percentage: 53%
- Depth percentage: 59.3%
- Pavilion angle: 40.75 degrees
- Crown angle: 34.5 degrees
- Pavilion depth: 43.1 %
- Crown depth: 16.2 %
Unfortunately Tolkowsky’s measurements do not include the girdle. The girdle is what secures the
stone in the setting, a sharp girdle is fragile and brittle. A very thin or larger girdle is necessary to
set a stone in any ring or jewelry piece.
A diamond's ability to reflect light determines its display of fire and brilliance. Diamonds are usually
cut with 58 facets, separate flat surfaces. These facets follow a mathematical formula and are placed
at precise angles in relation to each other. This relationship is designed to maximize the amount of
light reflected through the diamond and to increase its beauty.
In many cases the visual differences from one classification to the next are so small they may be
indiscernible to the naked eye. Science has produced sophisticated machinery that can measure
every aspect of a diamond’s proportions. It is this precision that allows such strict standards to be
defined. These guidelines give you the ability to not only make a selection regarding cut but to
understand the underlying science upon which it is based. In any case a brilliant and scintillating
diamond is ultimately the goal.
The lowest score becomes the overall cut grade. In order to classify each diamond with an overall
cut grade the lowest assigned grade for any individual characteristic is always used. For example:
If the table percentage falls within Ideal yet the depth percentage is in the Very Good range, the
diamond is classified as Very Good.
The proportions are not the same for every shape. Many of the diamond shapes require their own
guidelines in order to achieve maximum beauty. Due to the mathematical differences inherent in
different shapes, the table and depth guidelines are formulated to maximize fire and brilliance.
Although the numbers may be different, the goal is the same: a magnificent display that highlights
the individuality and character of each shape.
The girdle is the outer edge of a diamond.
When purchasing a diamond, select one with a girdle that is not Extremely Thin nor Extremely thick.
It usually has a frosted appearance. Many diamonds are also finished with a fully polished or even
a faceted girdle. This characteristic does not affect the value of a diamond and is often more a
reflection of the diamond cutter's preference. The girdle is rated in terms of thickness. Girdle size
is generally defined as Extremely Thin, Very Thin, Thin, Medium, Slightly Thick, Thick, Very Thick,
or Extremely Thick. The girdle can also be described as a range of these terms such as Thin to
Thick. Avoid either Extremes.
The culet is the bottom point of the diamond. In many cases this point actually has a very small facet.
The culet is referred to in terms that relate to the presence or size of this facet. The culet is generally
graded as None or Pointed, Very Small, Small, Medium, Slightly Large, Large, Very Large, and Extremely
Large. Smaller is more desirable.
This characteristic refers to the finishing or final polishing of the facets, or flat surfaces. Contrary to
common belief, diamonds are ground and polished, not chipped away, until they reach their final form.
Each facet is carefully fashioned by the diamond cutter to shine and be free from polishing imperfections.
The polish of a diamond is graded from Poor, Fair, Good, Very Good, to Excellent. When purchasing a
diamond, select one with a polishing grade of Good or higher.
This characteristic refers to the alignment and positioning of the facets, or flat surfaces. Each facet
should be carefully positioned by the diamond cutter in proper proportion and relationship to the other
facets. The alignment of each facet should be sharp and precise; improperly joined facet junctions can
make a diamond appear uneven. The symmetry of a diamond is graded from Poor, Fair, Good, Very Good,
to Excellent. When purchasing a diamond, select one with a symmetry grade of Good or higher.
For titanium tension settings select very good symmetry or better since the stone is more visible than in
more traditional settings.