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©2009 The Gemological Institute of America. All rights reserved.
GIA Cut Grade
Estimation Tables
for Standard Round
Brilliant Cut Diamonds
The following example tables provide guidelines for
estimating a GIA Cut Grade for round brilliant diamonds
with crown angles between 22.0 and 40.0 degrees, pavilion
angles between 38.8 and 43.0 degrees, and table sizes
between 50 and 67 percent. The ranges for these parame-
ters exceed the common proportion ranges seen in practice
today. Values for the other essential proportion parameters
are kept constant: star length (55 percent), lower half
length (80 percent), girdle thickness (3.0 percent), polish
and/or symmetry (VG or EX), girdle min/max (THN-STK),
culet size (NON-SML), and painting or digging out at
none or negligible. Variations in these fixed parameters
may result in a different final cut grade.
How to use these tables
These tables can be used to estimate the proportion cut
grade for a particular standard round brilliant, or they can
be used for general cut planning. The proportions provide a
primary component of the overall cut grade; other critical
factors include polish, symmetry, and verbal descriptions
of the girdle and culet (see, “Finish, Culet Size and Girdle
Thickness; Categories of the GIA Diamond Cut Grading
System” for details). The lowest grade for any category
establishes the final overall cut grade.
Example: Consider a round brilliant diamond with these
proportions: table 56%, crown angle 36.5°, pavilion
angle 41.2°, star length 55%, lower half 80%, medium
gir
dle, no culet, Very Good polish and symmetry, and
girdle thickness 3.0%. This example is one of many
proportion combinations that lie on the boundary between
Excellent and
Very Good cut grades.
Use the table at the top of page 5 for the 56% table size.
Find the 36.5° crown angle across the top and the 41.2°
pavilion angle down the side. Since the values for the other
three proportion parameters match the values used for these
tables, the grid box where the crown and pavilion angle
values intersect is the estimated cut grade (Excellent).
This cut grade falls close to the grade border between
Excellent and Very Good. If the average measurements also
lie close to one or more rounding boundaries, the slightest
difference between a measurement taken by GIA’s
Laboratory and one taken by a cutter can cause a difference
in the estimated cut grade. (See “Proportion Measurement:
Tolerances for the GIA Diamond Cut Grading System” at
http://www.gia.edu/diamondcut/pdf/0805_pg34_39.pdf)
For our example stone, the rounded pavilion angle is
41.2°. If 41.2° is derived from an unrounded value of
41.26°, it is within measuring tolerance of a value like
41.31°. However 41.31° would round to 41.4° and be
reported as such.
As shown on the table, a 36.5° crown
angle and 41.4° pavilion angle yields an estimated cut
grade of
Very Good. Although the estimated grade for
this combination of crown angle, pavilion angle, and
table size may seem to be Excellent, differences in
measurements can push a grade over rounding borders
and consequently over cut grade borders.
A two-dimensional table can only show variations with
respect to two variables; a number of such tables are given
here to provide the proportion cut grade dependence on
three parameters. The list provided here is not exhaustive.