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4 C's of Diamonds - Cut
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 ofthe 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 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 plane of the girdle
- angle of the pavilion facets to plane of the girdle |
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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
faces, 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.
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