When a round brilliant cut diamond has been cut to precision, it means that it
has been cut to mathematically correct proportions. This allows all the light
entering from any direction to be totally reflected through the top and
dispersed into a gorgeous display of color.

Even until recent years little importance was attached to the quality of
the cut. Before that time, grading was restricted solely to color and clarity.
Until the beginning of 20th century, the forms of diamond cuts developed
through trial and error as the optimal brilliance effect was pursued. Only
since 1910 have calculations been employed which consider the optical and
physical properties of diamond - such as hardness, light refraction and
dispersion, in order to achieve maximum brilliance through correct
proportions and symmetry.

This style of cut has probably experienced the longest and most intensive
history of development; it began as long ago as the first century B.C. and
only in the 20th century did it mature in the form of a round brilliant cut.

Uncut diamonds hide the unique optical properties of the cut stones.
Rough diamonds have a yellow, brown, or grey non-transparent skin which
covers them. Only cutting can transform the unattractive 'pebble' into that
crystal clear stone which, in clarity, luster and play of color surpasses all
other precious stones.

Most of the beauty of a cut diamond is in its amazing optical properties,
particularly in the way it bends light, and its play of color. The round brilliant
displays these properties in the cut state where all of the factors blend in
optical harmony with one another to create the highest degree of brilliance.

Color and Clarity are rarity factors given to us by nature. As the cut
reveals these qualities, it also serves as a beautifying factor that is influenced
by mans' skill alone.

Judgment of cut is based on one hand, on the impression gained at first
sight, in which beauty and brilliance are judged purely on personal standards.
Thereafter, attention must be directed to the proportions of the cut stone.
Here, facet angles and proportions must be tested for accuracy. The grading
of the quality of the cut is therefore always made up of two components: a
subjective opinion of the brilliance effect, and an objective judgment
determined by checking the craftsmanship and measuring the proportions.
In contrast, to color and clarity grading, where a natural degree of rarity
exists, in cut grading it is the manual skill of the stone cutter which is judged,
because it has a direct influence on how beautiful the stone is.

The quality of the brilliance is dependent on: - the proportions - the symmetry - external features

*Gemological Institute of America

The factors which contribute to the proportions are:
- girdle diameter
- table diameter
- total depth
- crown height
- thickness of the girdle
- pavilion depth
- angle of the crown facets to the plane of the girdle
- angle of the pavilion facets to the plane of the girdle
 

The more the cutter deviates from the mathematically correct proportions,
the more the brilliance is affected, and the lower is the quality of the cut.
However, a very slight departure from precise standards does not enable
any loss of brilliance to be seen even by the specialist. It is therefore normal
practice not to apply exclusively one value, but to extend this value upwards
and downwards within narrow tolerances. Only when a "Brilliant Cut" varies
beyond these limits of tolerance can it no longer be graded "excellent" but
only either very good, good, fair or poor.In the modern Brilliant Cut the requirements for maximum light emission,
through the table and for the greatest possible play of color on the upper
crown facets, are fulfilled by a mathematical blending of the proportions.

Maximum Brilliance:
The inter-play of luster, light refraction, total reflection, color dispersion
and scintillation. All this is the result of the practical experience and
craftsmanship of the cutter, along with his applied understanding of optical
law; for only when precisely calculated planes and angles are used in the
brilliant cut does the stone attain its' greatest possible beauty.

External Brilliance:
Luster, produced by the reflection of light on the surface of the facets.

Internal Brilliance:
Refraction of total reflection of light on the pavilion facets.

Dispersive Brilliance:
Splitting and scattering of light into its spectral colors.

Scintillation Brilliance:
The sparkle of the stone when moved, caused by light reflections of the
light source. It is the term used for the changing colors, the radiance and
sparkle of the rays when the stone is moved or when the light source
changes.

Total Depth:
The ratio of the crown to the pavilion, and the thickness of the girdle, are
the most important considerations when judging total height. Checking the
correct relationship of these proportions is far more important than
measuring the total depth alone.

The table:
The smaller the table, the more play of color will come through the
sloping crown facets.
The quantity of light reflection generated depends on the number, size and
symmetrical arrangements of the facets, as well as on the quality of the
polish.

Crown height:
Because of the steeply rising prices of rough diamonds, there has been a
tendency over the last few years to save weight when cutting rough. This
practice has led to round brilliants with a very shallow crown and the
resultant larger table. These shallow brilliants look larger than a precision cut
stone of equal weight, but the dispersion based on the color separation is
diminished because of the smaller crown facets. In a brilliant cut with a shallow
crown and large table, the major part of the light reflected on the pavilion
facets passes through the table without being scattered.

Spread Cut

Precision Cut

Girdle thickness:
The girdle is the dividing line between crown and pavilion. Its function is
to protect the edge of the stone from damage and fracture.
The girdle should be very thin, so that it is just visible to the unaided eye
as a light line. A thick girdle can adversely affect the color of a cut diamond,
and also diminish the light yield and therefore, reduce the brilliance (light
rays in the region of the girdle width are refracted into the air and not
totally reflected).

This applies to girdles left in a natural state, as well as to polished or faceted
girdles, which only reinforce the perfection of a well made stone if it is also
thin and even.
Completely or partially knife-edge girdles are dangerous, as they easily
fracture during setting of the stone and can thus produce nicks and cleavage
cracks.

Pavilion Depth:
In order to understand the important function of the pavilion facets, it
must be realized that the light falling from above into a brilliant can only be
reflected back by total internal reflection on the pavilion facets, if and only if
the angle between the pavilion facets and the girdle corresponds to the
mathematically correct angle.