167
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1. DESCRIPTION
The grading report will state if the diamond is of a natural
or synthetic origin. Most grading laboratories do not grade
synthetic diamonds. Those who do, issue a certicate that
looks quite different from the certicate they issue for natural
diamonds, and which states very clearly that the diamond is of
synthetic origin. Some synthetic diamonds will have a maker’s
mark, such as “GEMESIS”, laser-inscribed on their girdles.
Unfortunately, this identication mark can be polished off.
2. CARAT WEIGHT
The rst and easiest parameter to be determined is the weight
of a diamond. This is normally done by using an electronic
diamond scale. The unit of measurement is the carat (which
is equal to one-fth of a gram) and which is subdivided by
100 points. A 75-pointer diamond weighs three-quarters of
a carat. Diamond weights are normally rounded off to the
nearest point, ie, a weight of 1.696ct would be rounded to
1.70ct, and a 1.995ct would be rounded to 2.00ct. A weight
of 0.684ct would be stated as 0.68ct.
3. SHAPE AND CUTTING STYLE
Among the various shapes into which diamonds are
fashioned, the 58-facetted modern round brilliant is the
most popular, and also, per carat, the most expensive. A large
number of variations of the brilliant and other cutting styles
have been developed recently. Most of these variations have
been patented under specic brand names, eg, “Hearts and
Arrows” , “Hearts on Fire”, “Lily” and “Royal” cuts. There are
literally a hundred or more branded diamond cuts available
worldwide. Grading laboratories will normally not use a brand
name to describe the shape and cutting style of a diamond.
DIAMOND GRADING
CHAPTER 7
• Grading principles
• Evaluating the cut
• Fancy cut diamonds
• Fancy coloured diamonds
• Colour and
clarity enhancements
A PRACTICAL GUIDE TO DIAMOND GRADING
GRADING PRINCIPLES
The value of a diamond is mainly determined by its beauty and its rarity. It is generally accepted
that the more a diamond “sparkles”, the more beautiful it is. We will see in later sections that the
proportions of a diamond determine its sparkle.
The rarity of a diamond is determined by a number of factors, such as its size, colour and the size and
number of inclusions it hosts. In order to arrive at a diamond’s value, its relative rarity and beauty
need to be determined. We call this the grading process.
Today, most of the larger and better-quality diamonds are accompanied by grading reports that have
been issued by independent laboratories. It is up to the jeweller to understand these reports and to
be able to determine whether a specic report belongs to a specic diamond.
168
Chapter 7
4. MEASUREMENTS (DIMENSIONS)
This is normally done by using a millimetre gauge (the
Leveridge Gauge is the model best known) or modern
digital measuring devices which display readings of up to
one-hundredth of a millimetre.
No round diamond is perfectly round and a maximum and
minimum diameter needs to be determined. The depth
of the stone is important and is always stated last. For
example, 5.57 x 5.59 – 4.41mm.
It is important to be able to measure diamond dimensions
properly, even when mounted in jewellery, because these
dimensions can be used to identify the stone and/or to
calculate its weight. See Part III, page 240, on how to calculate
a diamond’s weight from its measured dimensions.
5. COLOUR
Diamonds come in many colours. The normal colour
range is from colourless to yellow or brown and 95% of
all diamonds fall within this range.
A colour grading scale developed by the Gemological
Institute of America (GIA) is the one most widely used. It
ranges from D (colourless) to Z (light yellow or brown).
Each letter represents a range of colour that is based on a
combination of tone (darkness or lightness) and saturation
(intensity of the colour). The combination is called depth of
colour, which is an indication of how noticeable a colour is.
Why not an A colour? When GIA developed its grading scale,
others in the industry were already using letters such as A,
double A or AAA, or terms such as “blue white” or “ne
white”. In order to have a truly unique system, GIA started its
colour scale with D (which is very appropriate for a diamond,
it can be said!).
• Method of grading
Diamonds are colour graded under specic lighting conditions
by comparing them with diamonds of known colour, called
master stones. The stones are compared with their tables
down in a special white display tray under daylight-equivalent
uorescent light. Colour grading is one of the most difcult
parts of diamond grading. As no two master sets are alike - and
because it is difcult to compare the colour of two diamonds
if they are not similar in size and cut - it may happen that two
graders have a different opinion on the colour of a stone.
• Fluorescence
Many diamonds glow with a visible colour when exposed to
ultraviolet light. This is called uorescence. About 35% of all
diamonds uoresce in mainly a faint or darker blue colour.
Other possible uorescent colours include white, yellow
and orange. Sunlight contains ultraviolet light and a yellow
diamond with strong blue uorescence may appear less
yellow and somewhat “milky” in direct sunlight.
A selection of modern diamond-cutting styles
Using a millimetre gauge to measure dimensions of a set diameter (left)
and a loose one (right).
Increasing colour grades in diamonds, with two fancy colours on the right.
A face-down diamond concentrates colour in the girdle and culet area.
Diamonds showing uorescence under ultraviolet light.
Round brilliant Oval
Pear-shaped Baguette
Emerald Princess square
Heart
Marquise
169
DiamonD graDing
6. CLARITY
The rarity of a diamond also depends on how free it
is of internal inclusions and external blemishes. These
imperfections are identied under 10x magnication and
plotted on the grading report. The diamond is then allocated
a clarity grade which is determined by the size, number, type,
positions and visibility of these imperfections.
A loupe is the most convenient magnier to determine
clarity, while a gemmological microscope is often used for
the grading of high-quality stones.
The table on the right gives the symbols that are used to
describe the clarity grade.
• Plotting symbols and abbreviations
The grading report will indicate the type and position of
imperfections that can be seen under 10x magnication. These
symbols are of great help in identifying a diamond and a jeweller
should know how to read a diamond plot. External character-
istics are normally indicated in green, while internal characteristics
are plotted in red. Herewith an example of such a plot and the
terminology normally used is given on the next page.
CLARITY GRADING SYSTEM
Flawless (FL)
- no internal or external imperfections
can be seen by the trained eye under 10x magnication
(used by GIA)
Internally Flawless (IF)
- no internal imperfections
under 10x magnication (used by EGL and others)
Very, Very Slightly Included
( VVS1, VVS2) - very
difcult to see with 10x loupe
Very Slightly Included
(VS1, VS2) - difcult to see
with 10x loupe
Slightly Included
(SI1, SI2, SI3) - easy to see with 10x
loupe, sometimes visible with the naked eye (GIA does
not use SI-3 grade)
Included
(I1, I2, I3) - imperfections visible with the
naked eye, seriously affecting the stone’s brilliance
(In Europe, the term “pique” is used for an included
stone and the symbols P-1, P-2, P-3 are used)
Relative size of inclusions for VVS1-I3 clarity grades
COLOUR GRADING SYSTEMS
Traditional terms Description
GIA
EGL
South Africa Europe
D nest white Jager
E (blue white) River colourless
F ne white
G
Top
Wesselton
face-up
colourless
H white Wesselton
I
commercial
white
Top Crystal
small stones
look colourless
face-up, large
ones are tinted
J top silver Cape Crystal
K
face-up a
faint colour
L silver Cape Top Cape is visible
M
N light Cape Cape
mounted
stones
O-R Cape Low Cape visibly yellow
S-Z dark Cape
very light yellow
Fancy
colours
darker than Z
Internally
Flawless
VVS1
VVS2
VS1
VS2
SI1
SI2 SI3
I1
I2 I3
SI-2
I-1
170
Chapter 7
TYPICAL INCLUSIONS IN POLISHED DIAMONDS, PHOTOS: EGL
External graining. Typical fracture in diamond (S1-2)
The same inclusion being reected three times. Zig-zag inclusions.Pinpoint inclusion.
Diamond octahedron in diamond.
Octahedral ghost cloud. Hair-like inclusions.String-like imperfections.
PLOTTING SYMBOLS AND ABBREVIATIONS
The grading report will indicate the type and position of imperfections that can be seen under 10X magnication.
These symbols are of great help in identifying a diamond and a jeweller should know how to read a diamond plot.
• Naturals and extra facets are always plotted, as they are of great
help in identifying a diamond.
• If the diamond is mounted, the metalwork is indicated by dashed
black lines.
External characteristics
Bearded girdle (Br)
Bruise (Br)
Cavity (Cv)
Chip (Ch)
Cleavage (Clv)
Cloud (Cld)
Crystal (Cr)
Feather (F)
Grain centre (Grncnt)
Natural indent (Nind)
Internal graining (Intgr)
Knot (K)
Laser drill hole (Ldh)
Needle (Ndl)
Pinpoint (Pp)
Twinning (Tw)
Abrasions (Abr)
Extra facet (Ef)
Natural (N)
Nick (Nic)
Pit (Pit)
Polish line (Pl)
Polish swirls (Psw)
Scratch (Scr)
Surface graining (Sgr)
Internal characteristics
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DiamonD graDing
7. CUT
The modern round brilliant cut has 58 facets, named and
arranged as shown herewith.
We know that the cut, ie, the proportions of a diamond, inuences
its sparkle. For many years, diamond-cutters have argued about
which set of proportions provide the best sparkle. With the
advent of branding, these arguments got worse because brands
normally claim that
their
proportions are superior.
In recent years independent gemmological laboratories
conducted extensive research on how effective diamonds
of different proportions reect and refract incoming light.
Various aspects such as brilliance, re and scintillation
were analysed through computer modelling and
observation testing. These laboratories each devised their
own proprietary system by which they grade the cut of a
polished diamond.
These systems also take into account aspects such as
the weight and durability of a diamond (ie, overweight
stones and those with extremely thin girdles), as well as
the quality of the finish (ie, the polish and symmetry of
the facets).
It is now known that every facet of a diamond contributes
to its overall “look”. Also, that the historically accepted
set of proportions used for “ideal cuts” are just one
of many sets of proportions that can produce maxi-
mum sparkle.
Furthermore, the type of light source, the angle of incidence
of light and the reection of large objects play signicant
roles in the overall appearance of the stone.
IMPORTANCE OF PROPORTIONS
The following sketches explain the importance of a diamond’s
proportions. Although this is an extreme simplication of the
way in which a diamond handles light, it allows the client to
quickly grasp the importance of a diamond’s proportions.
The proportions of a polished diamond are described below.
MEASUREMENT OF PROPORTIONS
Most jewellers use a manual reticule (microgauge) and/or an
electronic millimetre gauge to measure and then calculate
the proportions of a diamond. These proportions can also
be easily measured with a calibrated eyepiece that ts most
microscopes or with the proportion scope.
Modern grading laboratories employ a computerised optical
instrument to measure the proportions and to assign a cut
grade to polished diamonds.
With practice, a jeweller can estimate a diamond’s
proportions and evaluate its cut by using a variety of visual
observations, as described below.
ASSESSING PROPORTIONS
While most grading laboratories prefer to use their own
terminology in describing and evaluating the quality of a
diamond’s cut, they do, however, base their assessment on
the same parameters, which are:
I. The visual appearance of the stone,
II. Value aspects, and
III. The cutter’s craft
EVALUATING THE CUT OF A DIAMOND
Table
Star, 8 facets
Bezel, 8 facets
Upper girdle, 16
Crown - top view
Pavilion - bottom view
Lower girdle, 16
Pavilion main, 8 facets
Culet, not always facetted
Girdle
S
OUTH
A
FRICAN
IDEAL CUT
™
TOO
DEEP
TOO
SHALLOW
S
OUTH
A
FRICAN
I
DEAL
C
UT
™
T
OO
D
EEP
T
OO
S
HALLOW
S
OUTH AFRICAN IDEAL CUT
™
T
OO
D
EEP
T
OO
S
HALLOW
Ideal proportions. Too deep. Too shallow.
gird le diam eter
table %
crown height crown an gle
girdle
thickness pavilion total
pav ilion angle depth
depth
culet size
172
Chapter 7
I. VISUAL APPEARANCE
This is due to a combination of three optical effects, namely:
1. Brightness, ie, the total reection of white light.
2. Fire, ie, ashes of colour.
3. Scintillation, ie, the twinkling of areas of light and dark.
1. Brightness
This is mainly determined by the stone’s depth %, table %, its
crown angle and its pavilion angle. The latter two proportions,
with its girdle thickness, determine the stone’s depth %.
These parameters are measured or estimated as follows:
• Depth %
Is calculated by dividing the average
diameter of a round stone by
its depth, or the width of fancy
shaped diamond by its depth. This
percentage normally varies between
58-60%.
• Table diameter %
This parameter is measured with a calibrated micro
gauge under magnication. The largest table measurement
is divided by the average diameter. Opinion differs
as to the ideal table percentages. From 56% to 62% is
normally acceptable.
A quicker method is by observing the bowing of the line
running from the point of one star facet, along the table
edge and to the point of another star facet. See below.
• Crown angle and height
The higher the crown, the greater the crown angle. Most
diamonds have crown angles between 30-35 degrees.
Diamonds with crown angles less than 30 degrees can
be quite brilliant, but normally have extremely thin girdles
which are susceptible to damage.
Crown angles are estimated by looking at the prole of
the table and bezel facets and estimating the angle. See
sketches below.
The crown height is measured by a calibrated microgauge
and is expressed as a % of the diameter.
• Pavilion depth %
Is expressed as a percentage of average girdle diameter
and can be judged visually, or measured and calculated as
in the sketches on the following page.
Pavilion depth normally varies between 41-45%. In stones
with a pavilion depth of 40% and less (a shallow pavilion),
an unattractive, greyish reection of the girdle can be seen
under the table. This is called a
sh-eye
effect. It renders the
stone lifeless and dull. When the pavilion is too deep, the
diamond will be dark in the centre. This is called a
nailhead
.
Estimating the table % by using the bow method
ESTIMATING CROWN ANGLES
Fish eye: Grey reections of the girdle
just inside the table, due to incorrect
girdle thickness, table % and pavilion
angle. Gut grade poor to fair.
x 100 = depth %
A
B
A
B
Nailhead - a dark area in the
centre, resulting from too-steep
pavilion angles.
53%
58%
60%
63%
67%
Hand-held micrometer
Using the microscope to measure
table diameter (in mm) with a
hand-held micrometer
Table diameter = 2.8mm
45
0
90
0
30
0
34
0
25
0
173
DiamonD graDing
An incorrect pavilion depth also negatively affects the re
of a diamond.
2. Fire
The ashes of colour you see in a diamond are called its
re and are the result of white light being dispersed into its
various spectral colours. Fire is
difcult to assess, especially in
small stones, and is best seen
under a spotlight. The more
colour ashes, the better.
3. Scintillation
The ashes of light and the contrasting areas of darkness
that constantly change (sparkle) as the light source,
the observer or the diamond is moved, is called its
scintillation. The quality of scintillation is determined by a
combination of the stone’s sparkle and the pattern which
the contrasting dark and light areas form. The dark areas
are normally areas of light leakage (loss) or reections of
dark objects near the stone.
The pattern of ashes should be spread evenly and
balanced over the face of the stone and the contrast
between dark areas and bright ashes should be crisp.
Scintillation is best seen under uorescent light.
Please note
: The brightness, re and scintillation are
individually assessed and each described as either “
Excellent
”,
“
Very Good
”, “
Good
”, “
Fair
” or “
Poor
”.
II. VALUE ASPECTS
1. Weight ratio
A diamond’s physical dimensions in relation to its weight
are an indication of whether it was made “heavier” by
fashioning a too-thick girdle, a very large culet or incorrect
crown and pavilion angles. “Heavy” stones thus look lumpy
and face up smaller than those with the correct weight
ratio. On the other hand, a “spread” stone looks bigger
than it should and normally has a very thin girdle that
makes it vulnerable to damage. The prices of spread or
lumpy stones are thus adjusted downwards.
DETERMINING PAVILION DEPTH BY
IDENTIFYING THE TABLE REFLECTION
Look for small “bow-ties” and their position in the table
49% 50% or more
Brightness under uorescent light. Left: Excellent.
Right: With many dark areas, poor brightness.
Diamond with good symmetrical
scintillation (above left). Good
scintillation with “ne” sparkle (above
right). Old cut diamond with poor
scintillation. Note the large and
irregular reections (left).
43%
44.5%
1
4
3
4
1
3
2
3
45.5%
47%
2
3
1
3
3
4
1
4
Pavilion depth
Pavilion depth
Pavilion depth
174
Chapter 7
2. Girdle thickness
Is expressed as a percentage of the
depth measurements and is normally
described as “
Extremely
” or “
Very
Thin
”, “
Thin
”, “
Medium
”, “
Slightly
Thick
” or “
Extremely Thick
”.
A very thin girdle can break, affecting
durability, while a very thick girdle will
make the stone look smaller than it
should and could create fuzzy, grey
reections in the stone.
3. Culet size
Most fancy cuts have culets, while round brilliants may not.
The purpose of the culet is to protect the bottom of the
stone against abrasion. It is visually estimated and normally
described as “
None
”, “
Small
”, “
Medium
”, “
Large
”, “
Very
Large
” or “
Abraded
”.
III. THE CUTTER’S CRAFT
This parameter comprises two factors - polish and symmetry
- and gives an indication of the care and skill with which the
cutter manufactured the nal product.
1. The polish
The polish is described and graded as “
Excellent
”, “
Very
Good
”, “
Good
”, “
Fair
” or “
Poor
”.
Obvious polish lines on several facets will result in a fair grade,
while the presence of only faint polish lines in an inconspicuous
place will give a good grade. An abraded culet, bearded girdle,
pits on the surface and a very rough girdle are other aspects
that could inuence the clarity or polish grade of a diamond.
2. The symmetry
The symmetry is also graded as “
Excellent
”, “
Very Good
”,
“
Good
”, “
Fair
” or “
Poor
”.
This gives an indication of how symmetrically the facets are
aligned in the cut stone. Aspects such as an off-centre culet or
table, a wavy girdle or a large difference between minimum
and maximum diameters will inuence the symmetry.
CULET DESCRIPTION
None
No polished surface visible
Small
Barely visible under 10x magnication.
Medium
Octagonal outline is visible
under magnication.
Large
Just visible to the naked eye.
Very large
Octagonal outline is clearly visible to
the naked eye.
“Spread” stone with
extremely thin girdle.
“Lumpy” stone with
very thick girdle.
Ideal proportions
extremely thin
very thin
thin
medium
slightly thick
thick
very thick
extremely thick
Polishing scratch marks. Burn marks on a polished diamond.
Small percussion marks on
pavilion facet edge.
Culet and pavilion facet abrasion.
SYMMETRY CHARACTERISTICS IN
ROUND BRILLIANT CUTS
Irregular table Culet off-centre Girdle outline not
a perfect circle
Wavy girdle Naturals on crown
and pavilion
Table and girdle
not parallel
Crown and pavilion not
aligned
Facets do not meet
in point
175
DiamonD graDing
ASSIGNING AN
OVERALL CUT GRADE
In the table below a range is proposed within which each
proportion parameter should fall in order for it to be
considered either excellent, very good, good, fair or poor.
These ranges are rounded off to the nearest percentage point.
If more than 70% of the various proportions of a diamond fall
within a certain grade, that grade may be assigned as an overall
cut grade. It is possible that the individual proportions of a
diamond can fall into three or more different grade categories.
In such a case, average out the different grade categories to
arrive at an overall cut grade. For instance: if among the eight
different proportions there are 2 x excellent / 3 x very good /
3 x good grades, the overall grade should be very good. If the
diamond has 1 x very good / 2 x good / 3 x fair / and 2 x poor
proportion parameters, the overall cut grade should be fair.
Gemmological laboratories may differ slightly in their opinion
of these ranges and how to “average” an overall cut grade.
COMMENTS
Any other important characteristic which the grader
has noted will be indicated in the “comments” section
of the grading report. Typically, slight girdle bearding,
graining or minor nish blemishes are noted here. These
characteristics are useful in identifying a diamond and, if
severe, will signicantly inuence its clarity grade.
Perfectly round.
Please note:
• For a diamond to receive an overall excellent cut grade, its brightness and re should be high and the pattern of its face-up
reections must be, even with strong contrasts between bright and dark areas, and;
• Its symmetry and polish must also be of an excellent grade.
• A software program called
Facetware
is available at www.diamondcut.gia.edu that allows users to estimate the overall cut
grade once the stone’s proportions have been measured.
Cut grade Poor Fair Good Very good Excellent Very good Good Fair Poor
Total depth % <52 52-54 54-56 56-58 58-65 65-67 67-69 69-71 >71
Table % <44 44-49 49-51 51-53 53- 62 62-65 65-68 68-72 >72
Crown angle <26 26-28 28-31 31-32 32-36 36-37 37-38 38-42 >42
Crown height % <7 7-9 9-11 11-13 13-15 15-17 17-19 19-21 >21
Pavilion depth % <39 39-40 40-41 41-42 42-45 45-47 47-48 48-49 >49
Pavilion angle <38 38-39 39-40 40-42 42-43 43-44 44-45 45-46 >46
Girdle
thickness %
0.5-2.0 2-2.4 2.5-3.5 3.5-4.5 4.6-5 5.5-6.5 6.5-9 >9
Girdle
description
very thin thin thin-med
slightly
thick
thick
very
thick
extremely
thin
extra
thick
Culet size
extra
large
very
large
large none-med none-small none-med med
very
large
extra
large
FANCY CUT DIAMONDS
The rules for grading fancy cut diamonds are similar to those
of round brilliants, except for the following differences:
1. Inclusions are easier to see in step cuts than in
fancy brilliants.
2. Colour is often concentrated in the tips of marquise,
pear and oval. If it looks darker face-up than in the
table-down position, the colour is lowered to that of
the face-up position.
Out of round.
176
Chapter 7
3. Table width % is calculated as:
table width B
------------- x 100 = table %
stone width A
4. Crown angle is estimated as with round brilliant cuts,
but by looking at the stone’s lengthwise prole.
5. Total depth % and pavilion depth %
First measure and calculate the stone’s total depth as:
stone depth (measured)
------------- x 100 = total depth %
stone width (measured)
View the stone in prole and estimate the crown height
and girdle thickness. Then calculate as follows:
Total depth % - (estimated crown height % + estimated
girdle thickness %) = pavilion depth %
Describe the total depth % and the pavilion depth % as
“Acceptable”, “Slightly shallow”, “Very shallow”, “Slightly
deep” or “Very deep”.
6. Bow ties and bulges
For step cuts, describe their bulge factor:
For fancy brilliants, describe the bow-tie:
7. The length to width ratio
This is measured as shown below, and calculated as follows:
length
------- = ratio
width
This ratio is expressed as, for example 1.52 : 1 (note: this
ratio means that the length of the stone is 1.52 times
longer than its width).
The length to width ratio is described as “
slightly
noticeable
”,“
obviously long
” or “
obviously short
”, using
the table below.
8. Culet size and position
Fancies often have an elongated culet. Observe only the
width of the culet and assess the same as with rounds.
A
B
Bow-tie effect
The bulge of the
pavilion of a fancy cut
diamond.
Obvious bulges will
detract from the value.
Width
B
Length
PREFERRED LENGTH TO WIDTH RATIO
FOR VARIOUS FANCY SHAPES
Shape Preferred Too long Too short
Emerald 1.50-1.75 : 1 >2.00 : 1 1.25-1.10 : 1
Heart 1.00: 1 >1.25 : 1 <1.00 : 1
Triangle 1.00 : 1 >2.50 : 1 <1.00 : 1
Marquise 1.75 - 2.25 : 1 >2.50 : 1 <1.50 : 1
Oval 1.33-1.66 : 1 >1.75 : 1 1.25-1.10 : 1
Pear 1.50-1.75 : 1 >2.00+ : 1 <1.50 : 1
Culet too low Culet correct Culet too high
Total depth % =
Depth = 8,55 mm
Width = 5,46 mm
5.46
x 100 = 64%
8,55
Crown 13% (est)
Girdle 3% (est)
Pavilion depth =
64 - (13+3) = 48%
Culet Culet
177
DiamonD graDing
Diamonds that exhibit colours other than yellow or brown -
and those yellow and brown stones with a colour more intense
than Z - are called fancy coloured diamonds or “fancies”. They
also come in other more unusual colours, such as red, green,
purple and blue, etc. In natural diamonds the intensity of these
unusual colours is normally quite weak. However, if these
unusual colours are intense, they are normally indicative of a
synthetic, man-made diamond or of a natural diamond whose
colour has been enhanced in the laboratory. See the section
on enhancements at the end of this chapter.
NATURAL FANCY
COLOURED DIAMONDS
Diamonds with a natural red, pink, blue or green colour are
extremely rare and fetch astronomical prices. These colours
are normally quite pale compared with the intense yellow
some natural diamonds exhibit. In the ‘90s, an exceptional
red diamond was sold for US$92 600/ct while vivid fancy
pinks from the Argyle mine in Australia are annually put on
auction and bought by collectors at prices ranging from
US$30 000-200 000/ct.
Small pale pinks and blues are readily available and have
become a fashionable item in ne jewellery.
Natural fancy yellow diamonds are more readily available.
They exhibit a vast range of colours - from “canary” (pure
yellow) to orangy-yellow, greenish-yellow to brownish-
yellow, which makes pricing them a specialist art. The pure
yellow stones with no other colour modiers are the most
expensive. Half-caraters of such colour can be bought from
US$2 000-5 000/ct.
Brown diamonds have been produced in large volumes at
the Argyle mine in Australia. They come in colours described
as “
beer bottle
”, “
champagne
” or “
cognac
”, which are quite
affordable. Combined with white diamonds, they make
beautiful jewellery, especially for men.
NOMENCLATURE FOR
FANCY COLOURS
Grading laboratories have developed their own individual
system for describing and grading fancy coloured
diamonds. This is confusing, because certicates from
two laboratories may differ widely in their description
of the same stone. Fancy coloured diamonds should not
be valued on the description of one laboratory only, but
rather on what the eye sees.
The following colour grading system for “fancies”
is preferred.
Yellow and brown colours
Yellow or brown coloured diamonds with a colour stronger
than the Z master stone is described with a “fancy” prex
and with increase in colour as follows: “
Fancy Light
”,
“
Fancy
”, “
Fancy Intense
”, “
Fancy Dark
”, “
Fancy Deep
”,
“
Fancy Vivid
”.
9. Symmetry aspects
Because of the many shape variations possible with fancy
cuts, there are a few extra symmetry issues that need to
be judged under X10 magnication, such as:
• the position of the culet;
• uneven wings and shoulders;
• non-parallel edges;
• and uneven bulges.
Please note:
Trust your instinct. If you like what you see
when observing a fancy cut diamond face-up, believe your
eyes. What you like, others will like too.
FANCY COLOURED DIAMONDS
A selection of natural fancy coloured diamonds (rough).
Uneven fancy cut.
178
Chapter 7
For example : Fancy Light greenish-yellow natural diamond,
or, Fancy Intense orangy-brown synthetic diamond.
EGL South Africa describes brown diamonds as
“
Champagne
” and indicates the intensity of colour on a
scale ranging from C-1 (light) to C-7 (very dark).
All other colours
Diamonds with a colour other than yellow or brown
normally exhibit a colour of much less intensity. A
diamond’s characteristic colour, ie, blue, pink, green, steel,
purple, etc, is modied by the following terms: “
Faint
” , “
Ver y
Light
” or “
Light
”.
For example: Very Light green fancy coloured diamond,
or Faint blue fancy coloured diamond.
COLOUR ENHANCEMENT
Intense yellow, green, blue and even pink colours can be
induced in a natural diamond by a combination of radiation,
heat treatment and/or special surface coatings. Such stones
cost a fraction of those with a comparable natural colour.
Also, the colour of a yellowish-brownish type IIb diamond
can be removed by a high-pressure/high-temperature (HP/
HT) process, rendering a beautiful white stone.
Fortunately, most gem laboratories can identify the
articial nature of a diamond’s colour.
Some grading reports will indicate NCNA, an acronym
for Natural Colour Not Authenticated. This means that
the laboratory has not established whether the colour is
natural or not. It is therefore of prime importance to have
coloured diamonds certied by a reputable laboratory
before they are sold to the public.
Please note:
• Be careful with pale green or pale blue diamonds - it is
difcult to distinguish a pale natural colour from a pale
colour induced by man. Natural vivid greens and blues
are extremely scarce and, if available, will most probably
be the result of laboratory enhancement.
• You should be aware that some fancy coloured diamonds
which have been treated - especially the yellows, browns
and blues - will change colour under a jeweller’s torch. A
normal repair job like tipping claws could cause a blue
stone to turn green, resulting in a very unhappy client.
BLACK DIAMONDS -
fact or fantasy?
Experts have long believed that there are no true
black diamonds and that they exist only in folklore and
detective stories. There are, however, three kinds of
diamonds that are often referred to as black.
One type is actually very dark brown or a gunmetal-
coloured diamond with so many specks of dark mineral
inclusions (believed to be graphite) that they appear
black to the unaided eye.
The other type is a dark yellow or dark brown diamond
that has been articially irradiated to produce a dark
greenish diamond that looks black.
The only true black diamond is one which does not
transmit any light. Even the thinnest sliver (when held
up to a very brightest source of light) will not transmit
any light. Such diamonds, if untreated, are extremely rare.
Because black diamonds are extremely difcult to polish,
they often show polishing drag lines and cracks across
facets that are visible to the naked eye. Their girdles are
normally chipped.
V. COLOUR AND CLARITY
ENHANCED DIAMONDS
CLUES FOR DETECTING
TREATED DIAMONDS
• A medium to dark, vivid green, blue- green or blue
body colour.
• Darker green or brown naturals or surface spots.
• Umbrella effect - a coloured pattern following
facet junctions.
• Disc-like inclusions.
• Colour zoning in the culet.
Please note:
Natural blue diamonds will conduct
electricity. Treated blues do not.
179
DiamonD graDing
CLARITY ENHANCEMENT
There are techniques available to improve a diamond’s
clarity, for instance:
• Laser drilling
A laser beam is used to drill an extremely thin tube into
a dark inclusion or a stained crack. A strong acid, which
is forced down the tube, dissolves the inclusion or stain,
rendering the imperfection less visible. The laser hole is
often lled with a colourless resin. Grading laboratories
will disclose whether a diamond has been laser drilled to
improve its clarity. Laser drill holes are quite easy to detect
under magnication.
• Fracture lling
Modern techniques are availably whereby a special glassy
resin with a RI similar to that of a diamond and with a
low melting point (similar to the material used to repair
windscreens) is sucked under vacuum into cracks that reach
the surface of a polished diamond. Under the microscope,
it requires some expertise to identify small bubbles that
may be trapped in the ll material. By using dark eld
illumination a colourful feature, called the “ash effect”, can
be seen which is indicative of a fracture-lled diamond. Such
lling makes a crack less visible to the naked eye and may
improve the clarity of a diamond by one or more grades.
Most gemmological laboratories will not grade fracture-
lled stones, as the treatment is not permanent. A
goldsmith’s ame may cause the ll material to melt and
leak from the crack.
Please note:
The diamond grading principles as described herein
are guidelines only. They will assist the jeweller in
assessing a grading report, as well as in the valuation
of diamonds. The jeweller must realise that the grading
report is worth only as much as the reputation of the
organisation which issued it. Also, that the diamond
itself is the only real thing of value.
Diamond grading is a specialised skill and much
experience is needed before a true grade can be
attached to a particular stone. In professional grading
laboratories, more than one grader must independently
arrive at the same grade before a report is issued.
The author assumes no responsibility if, by using the
guidelines herein, losses are suffered in the valuation of
diamonds and other precious gems.
Laser drill hole into an acid
wash inclusion.
Laser drill holes in a diamond.
Blue colour ash in a
fracture-lled diamond.
Orange ash in a
fracture-lled diamond.