GIA's Color Grading
Interest in colored diamonds
has grown considerably in recent years. For example,
requests for natural- color
colored diamonds services at the Gemological Institute of America (GIA)
increased 68% between 1998 and 2003. Numerous factors have led to this increase
including new mine discoveries, innovations in cutting and increased marketing
efforts. Realizing the growing importance of these diamonds, GIA published an
article in 1995 titled "Color Grading of Colored Diamonds in the GIA Gem Trade
Laboratory" in GIA's 30, NO. 4, pp. 220-242) describing its grading system.
METHODOLOGY OF THE SYSTEM
The first step in grading
the color of a colored diamonds is to determine the faceup color that is
"characteristic" or most representative of the color to be graded (figure 1).
The grading then consists of comparing that characteristic color of the diamond
being graded to a series of known reference diamonds.
GIA's COLOR GRADING
These reference diamonds
frame areas in the GIA colored diamond color space. The laboratory uses an
extensive set of colored diamonds for comparison to those being graded. These
reference diamonds are located at key strategic locations throughout the GIA
color space. They may mark a grade boundary, color description boundary, or
both. The same diamonds are always used and their designation does not change.
By bracketing the color for its different attributes, the diamonds being graded
is placed in a "box" in color space (Figure 2). Each of these "boxes" has a
color grade and description. Therefore, once a diamond is located in an area of
GIA's color space, the corresponding description is assigned. As is the case in
the other grading categories, a color grade and a color description by design
refers to a range of similar color appearances, not to a single
appearance.
This methodology must be
applied in a consistent way under standardized conditions to have repeatable
grading results. Our research has shown one cannot grade consistency without
known references and standard procedures. If one does not make the observations
under the same controlled conditions the likelihood of obtaining a reproducible
result is diminished.
To control the surroundings.
grading is performed in a viewing box manufactured by GretagMacbeth. Standard
fluorescent light bulbs manufactured by GretagMacbeth are used that have a color
temperature of D65. Colored diamonds are only graded under this one light source
as color appearance can change under this one light source as color appearance
can change under different light conditions and reduce consistency. The diamond
being graded is placed in a Grooved white viewing tray next to the reference so
that they are in the same face-up orientation in the same location in the
viewing box.
During the comparison, three
attributes are analyzed: The relationship between the diamond color and
reference and the diamond being graded in terms of lightness to darkness - tone;
the relationship of strength or purity of color - saturation and the hue - that
aspect that gives a color its name such as red, green, and or blue. Based on the
analysis of the attributes, another reference is chosen and the same procedure
is followed. This continues until the grader has placed the diamond being graded
into the appropriate area in the color space.
Fig.2: This
illustration indicates how a portion of color space can be divided into "boxes."
Three volumes are shown. By means of the color comparison and bracketing
process, the characteristic color of a faceted diamond can be located within one
of these volumes, or boxes, each of which has been assigned a color grade and
description.
Fig 3. This
grid illustrates in two dimensions (lightness on the vertical axis and
saturation on the horizontal axis) the relationship of GIA's colored diamond
color grades. The lighter, least saturated diamonds are located in grades such
as faint, very light, and light in the upper left portion of the grid whereas
the medium to dark, stronger colors are located toward the lower right in grades
such as fancy deep and fancy vivid.
COLOR GRADES
The grades used for
classifying colored diamonds are: Faint, Very Light, Light, Fancy Light, Fancy,
Fancy Dark, Fancy Deep, Fancy Intense and Fancy Vivid. These grades represent
the combined effect of a range of tone; the lightness or the darkness
saturation; the strength, richness or purity; on a color. The two-dimensional
grid in he Figure 3 illustrates the relationship of and the relative depth of
color associated with these grades. The vertical axis represents the tone
attributes transitioning from light at the top to dark at the bottom. The
horizontal axis representing saturation transitions from weak colors on the left
towards stronger colors on the right. Such as a grid helps to visualize the
depth of a color relationship among the color grades. For example, a color in
the Very Light is relatively light and weak in appearance. Those in the Fancy
Deep range are medium dark and moderate to very strong. Fancy intense diamonds
are of similar strength as those graded fancy deep but are lighter. The
tone/saturations chart in Figure 4 illustrates the range of appearances
associated with pink diamonds in these grades.
Fig 5. These four blue diamonds are similar in saturation but they differ in
tone; that is they become progressively darker from left to right. These
differences are, at times, misinterpreted as increasing in saturation. Photo:
Harold & Erica Van Pelt.
Fig. 6: Colored diamonds located near a fancy-grade boundary may appear
similar yet be described differently in GIA's system. These two diamonds are
similar in tone but differ slightly in saturation. Because they are near a grade
boundary the marquise is described as Fancy blue and the slightly more saturated
pear is described as Fancy Intense blue. Photo: Harold & Erica Van Pelt.
Fig 7. The two diamonds on the left may initially appear to differ more than
they actually do since observers have a tendency to try to make distinctions.
When a diamond (far right) of different tone and saturation is placed next to
them the similarities of the first two become more clearer. Photo: Harold &
Erica Van Pelt.
COLOR DESCRIPTIONS
The location of a color in
GIA's system may be further refined through the color description that is
reported in addition to the fancy grade. The color descriptions associated with
a grade are related to the color attributes hue, tone, and saturation, of
that grade; i.e., whether the grade is relatively light or dark, weak, moderate,
or strong. This practice follows the convention of color-naming systems used in
other disciplines.
One of the basic premises in
the system is that warm colors such as yellow or orange
appear increasingly brownish
or brown as they become darker and/or weaker; For cool colors such as blue, the
appearance transitions through grayish and gray. With this in mind, fancy grades
that are defined as medium to dark in tone and weak to moderate in saturation
will include grayish/gray or brownish/brown terms. This applies, for example, to
grades such as Fancy Light, Fancy, Fancy Dark and Fancy Deep (See figure 3).
Terms associated with the
grades are strongly colored, such as Fancy Intense or Fancy Vivid, have only hue
names such as yellow, purplish-pink or blue. Conversely, all the descriptions in
Fancy Dark, representing colors that are relatively weak and dark have a
grayish/gray or brownish/brown component. The grade of Fancy, which is medium in
tone and low to moderate in saturation, has the widest range of descriptions
associated with it; i.e., descriptions can range from a hue name to one where
gray or brown predominates. Because there are fewer discernable color
differences when colors are pale, fewer color names are used in grades such as
Faint, Very Light, and Light.
Fig 8. If a diamond is not compared to other known references it is difficult
to judge what attribute is affecting appearance. "Warm" color pink diamonds are
often considered weaker, i.e. browner, unless compared to graded brownish/brown
diamonds. Far left, a pink diamond (left) is shown next to an orangey-pink
diamond. In this comparison, it is easy to misinterpret the orangey-pink
appearance as brownish. On the right side, the same orangey-pink diamond is
placed next to a brownish-pink marquise - and the difference in color appearance
is apparent. Photos: Elizabeth Schrader and Don Mengason.
SUMMARY OF IMPORTANT
ASPECTS WHEN ASSESSING COLORED DIAMONDS.
While it is very important
to maintain consistent methodology and viewing geometry to understand the color
diamonds color grading, it is also important to understand some of the basic
premises behind the system. If some of the basic points of the system are not
understood the user can be confused or arrive at incorrect results. For example:
Color diamonds are only
graded face-up. The strength of the body color as seen through the pavilion is
not considered nor observed when grading.
The overall face-up
appearance is observed when grading.
A highly brilliant
diamond, i.e., one with a lot of "life" does not necessarily result in it
having a better color grade since this attribute does not contribute to the
depth of color.
Darker colors are not
necessarily strong colors; i.e. a fancy yellow diamond may be darker than
other fancy yellow diamonds but still not have the strength to be graded Fancy
Intense. Figure 5 illustrates this situation using blue diamonds.
The strength of color is
not the same as how well the color is distributed. A color may be "laid out"
very evenly but not have a better color grade. Conversely, a diamond may show
color in a limited area but, if that color is strong, it will receive the
grade reflecting that strength. The use of the distribution entry, even or
uneven, on the grading report reflects this condition.
The strength of the
face-up color is judged similarly regardless of the cut, quality, or cutting
style.
It is important to
understand the entire range in which colored diamonds occur. These are a number
of situations in which the value of this is seen. For example, two diamonds with
different color grades may not appear very different if they are near a grade
boundary (figure 6). Two diamonds with the same grade may look noticeably
different if they are at opposite sides of the grade range (figure 7). The
incorrect attribute may be attributed to a color appearance if the full range of
appearances is not understood. Figure 8 shows an orangey-pink diamond whose hue
could be confused with the appearance of lower saturation and described as
brownish-pink if the full range is not understood.
Colored diamonds are
associated with some of the storied diamonds in history. The opportunities for
buying and selling them have never been greater. A good understanding of how to
judge fancy colors is an integral part of successfully trading such stones.