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In order to assess the cut of a diamond, diverse aspects are
kept in view. The following main proportions are measured by GIA in order to
assess its cut. It has to be noted that the below mentioned parameters are
listed usually for diamonds certified by renowned laboratories like the GIA and
AGS. Amongst the various parameters those of girdle thickness, culet size and
finish are of high importance as these help estimate the polish and symmetry of
a diamond. In the case of round brilliant stones however, various other factors
are also kept in mind.
Average stone diameter
The average diameter of a stone is an important parameter
for measuring diamond cut especially in round stones. The diameter in these
stones is measured at the girdle and is the greatest measurement of width. This
parameter also forms the basis of all other measurements in a diamond. It is
considered being equal to 100% of width of the stone.
Size of table
As the table is the largest facet of a faceted diamond and
most of the light reflected from the pavilion passes through this facet, it
forms an important parameter for measuring the overall proportion of the stone.
A large table enables to retain larger diamond weight from the rough. However,
while deciding the size of the table, importance also needs to be given to
maintaining ratio and proportion with other facets in the crown as all these
together enhance the brilliance of a stone. In an ideal grade diamond, the
table size is in proportion to the average stone diameter.
Height of crown
This is calculated by measuring the percentage of the
average crown height relative to the average diameter of the diamond. It is
measured from the table plane to the intersection point of the bezel facet and
the girdle in case of round brilliant diamonds.
Crown angle
This measurement is made using a diamond loupe or a
microscope. Diamond cutters usually maintain a crown angle of 34.5 degrees in
order to give a larger yield of diamonds. Also the table is cut closer to the girdle.
Height of pavilion
As most of the light that passes through the front of the
stone reaches the pavilion, from where it is reflected back through the crown,
this is an important parameter. The depth of the pavilion thus determines
brilliance and an ideal stone would have a pavilion depth of 43% of average
diameter. A shallow pavilion less than 37% of average diameter, will make the
stone appear extremely dull creating an appearance known as the ‘fish stone'.
Girdle thickness
The thickness of the girdle plays an important role in
determining the amount of light that will reflect from a stone. An adequate thickness is required in girdles
as it is the meeting point of the pavilion and crown and forms the region where
the stone will be held in a jewelry setting. A thin or ‘knife edge' girdle can look
attractive, but make a stone brittle. However an excessively thick girdle can
reduce brilliance as much of the light can be lost in it. Statistics state that
for stones that are a carat or over, a girdle thickness of 1.5% (of average
diameter) be designed. On the other hand, stones of half a carat can have
girdle thickness of approximately 3%.
Culet size
This is also of importance and as per the AGS, a pointed,
very small or medium culet is most desirable. Absence of a culet is also
desirable in the round brilliant diamond.
While ideal stones would as the name implies, make the
perfect purchase, any other well cut diamond is as priceless. As long as a
diamond is cut proportionately giving high brilliance and fire, it can be opted
for.
Grading parameters of different laboratories
GIA is highly renowned for its grading and has five
different grades. These are excellent, very good, good, fair and poor. It
grades stones on the parameters of the brightness of a diamond, its
scintillation and fire, diameter/weight ratio, polish, symmetry and durability.
On the other hand, AGS grades diamonds as Ideal, excellent, very good, good,
fair and poor. It considers eleven factors while evaluating cut. These are
scintillation, contrast, fire dispersion, brilliance, light leakage,
diameter/weight ratio, tilt, culet size, durability, polish and symmetry.
Tools and technologies for evaluating diamond cut
GIA uses its Facetware technology for grading stones. AGS on
the other hand, uses software like the DiamCalc and AGS ASET (Angular Spectrum
Evaluation Tool). Other tools frequently used by gemologists include the
Desktop ASET, Handheld ASET and Ideal scope. The DiaVision by Sarin is also
popularly used to measure and grade both round and fancy shaped diamonds. The
software also gives information on cut planning for rough diamonds and how to
improve the cut of a diamond through recuts. Besides the above tools, the
environment in which a diamond is studied is also important. While many
gemologists highlight the importance of a white background for grading cut,
experts from GIA state that a neutral gray background is more preferable as it
returns an appearance closer in similarity to most mountings.
Finally it is not necessary that a diamond with a higher cut
grade appeals more than one with a lower cut grade. This is because buyers may
like certain patterns of appearance in diamonds of a lower grade too. With the
diamond market becoming overly stringent, more complex rules for measuring and
evaluating continue to emerge. Thus while in previous times, diamond cutters
mainly considered the final weight of the stone as the main parameter, today
standards have risen.
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