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You are here: Physical > Grading Gemstones > Coloured Gemstone Grading

Faceted Gemstone Grading


Much like a diamond, the various combinations of a stone's cut, colour, clarity, and carat weight, along with availability and accessibility, determine rarity. You will find a brief description of those features here. These explanations are based on the system for gemstone evaluation established by the Gemmological Institute of America (GIA).


Colour

Colour is the most defining characteristic of a gemstone. Most gemstones occur in a spectrum of colours, while some have more restricted palettes. Since the colour phenomenon in gems is very complex, experts analyse the chromatic options in terms of three essential components: hue, tone, and saturation. Hue is the basic or the primary colour of the gemstone, tone refers to the lightness or darkness of the colour and saturation refers to the purity or intensity of the primary colour.

As a general rule, a bright, intense, pure, rich and vivid colour is always best. The most valuable gemstones are those that exhibit a pure colour with only slight hues of other colours in addition to their primary colour.

The primary colour should be medium, neither too dark nor too light.

While buying gemstones you need to be aware of colour ranges and objective value assessments, but it’s best to rely on your eyes and heart. Select your gemstone by trusting your instincts; if a particular colour speaks to you, by all means listen!

Hue

Hue is described as the shade, tint or sensation of a colour. The GIA has 31 Hues and the complete GIA GemSet® has 324 sample hue colours with varying Tones and Saturations. In addition we put RGB (Red Green Blue) and HEX codes for your reference.
Key   Code   Name   HEX   RGB
R   R   red   FA0303   R:250 G:3 B:3
oR   oR   orangy red   EE2102   R:238 G:33 B:2
  RO/OR   red-orange or orange-red   FB3E00   R:251 G:62 B:0
rO   rO   reddish orange   FE6000   R:254 G:96 B:0
O   O   orange   F87B00   R:254 G:96 B:0
yO   yO   yellowish orange   F6A400   R:246 G:164 B:0
oY   oY   orangy yellow   F7CD00   R:247 G:205 B:0
Y   Y   yellow   F8F800   R:248 G:248 B:0
gY   gY   greenish yellow   CEE600   R:206 G:230 B:0
YG/GY   YG/GY   yellow-green or green-yellow   B0DD00   R:176 G:221 B:0
styG   styG   strongly yellowish green   7BCE00   R:123 G:206 B:0
yG   yG   yellowish green   4DC100   R:77 G:193 B:0
slyG   slyG   slightly yellowish green   26C200   R:38 G:194 B:0
G   G   green   00C000   R:0 G:192 B:0
vslbG   vslbG   very slightly bluish green   00BE26   R:0 G:190 B:38
bG   bG   bluish green   00BA4A   R:0 G:186 B:74
vstbG   vstbG   very strongly bluish green   00B96F   R:0 G:185 B:111
GB/BG   GB/BG   green-blue or blue-green   00B893   R:0 G:184 B:147
vstgB   vstgB   very strongly greenish blue   09B4B4   R:9 G:180 B:180
vslgB   vslgB   very slightly greenish blue   0A59AA   R:10 G:89 B:170
B   B   blue   0916A5   R:9 G:22 B:165
vB   vB   violetish blue   0808A0   R:8 G:8 B:160
bV   bV   bluish violet   7B0AB4   R:62 G:8 B:170
V   V   violet   7B0AB4   R:123 G:10 B:180
bP   bP   bluish purple   C70BC7   R:199 G:11 B:199
P   P   purple   D40CB1   R:212 G:12 B:177
rP   rP   reddish purple   E90A9E   R:233 G:10 B:158
RP/PR   RP/PR   red-purple or purple-red   F50E80   R:245 G:14 B:128
strpR   stpR   strongly purplish red   F80E5C   R:248 G:14 B:92
slpR   slpR   slightly purplish red   F20E33   R:242 G:14 B:51
R   R   red   FA0303   R:250 G:3 B:3
Pk   Pk   pink (exception)   FFDFDF   R:255 G:223 B:223
Brn   Brn   brown (exception)   B7770D   R:183 G:119 B:13

Tone

Tone represents the depth of colour, ranging from colourless to black. In general, as light colours become darker they become more saturated. But as a colour becomes very dark, verging on black, tone increases and saturation decreases.  In other words, tone is described as the relative lightness or darkness of a Hue. Gemstone tone is described as ‘light’, ‘medium-light’, ‘medium’, ‘medium-dark’, and ‘dark’. Medium-light to medium-dark tone is considered as most valuable range.

Although from the tone scale given below you could think of midrange tones (such as 5) to be gray, this is not the case. Instead think of it as white or black mixed in with the pure hue.

 Hue, Tone and Saturation are associated with each other and play very crucial role in determining the gemstone’s colour. The more intense the colour, more is the value. This does not mean darker, but intense.

Key   Scale   Code   Name   HEX   RGB
c   0   c   Colorless or White   FFFFFF   R:255 G:255 B:255
ex1   1   ex1   Extremely light   F8F8F8   R:248 G:248 B:248
v1   2   v1   Very light   F0F0F0   R:240 G:240 B:240
l   3   l   Light   E8E8E8   R:232 G:232 B:232
ml   4   ml   Medium light   D6D6D6   R:214 G:214 B:214
m   5   m   Medium   C5C5C5   R:197 G:197 B:197
md   6   md   Medium dark   A3A3A3   R:163 G:163 B:163
d   7   d   Dark   888888   R:136 G:136 B:136
vd   8   vd   Very dark   646464   R:100 G:100 B:100
exd   9   exd   Extremely dark   424242   R:66 G:66 B:66
bl   10   bl   Black   000000   R:0 G:0 B:0

 

Tone Scale

           
Tone Scale
1 2 3 4 5 6 7 8 9 10
           

Saturation

Saturation is described as the strength or purity of a Hue. The GIA Saturation Scale is from 1 to 6. The lower numbers such as 1, 2 or 3 of warm colours such as red, orange or yellow and tend to look brownish and the cool colours such as blue and green tend to look greyish. Level 4 no longer shows either greyishness or brownishness, while neither is strong or weak. Level 5 is strong and level 6 being vivid, almost over coloured.


Scale   Code   Name
1   gr(br)   greyish (brownish)
2   slgr   slightly greyish (brownish)
3   vslgr   very slightly greyish (brownish)
4   mst   moderately strong
5   st   strong
6   v   vivid

Saturation scale (medium dark stone)

           
1 2 3 4 5 6
           

Clarity

There is no universally accepted standard for grading the clarity of coloured gemstones. Although the GIA coloured stone clarity grading system is a notable attempt to address the issue, it is still the subject of a great deal of discussion and debate.

Each gemstone is comprised of a unique combination of trace minerals. These create tiny fractures or mineral flaws called “inclusions.”  Under magnification or by their careful unaided eye, gemmologists take note of inclusions as well as “blemishes” or surface irregularities in gems. 

As with diamonds, the type and location of inclusions are more important than the fact that inclusions exist in the gemstone. It is important that any inclusions do not penetrate too deeply into the stone, because this may affect the gem’s durability, causing it to crack or break. Deeper colours can mask imperfections better, so it is more important in lighter coloured stones to select those with fewer blemishes or inclusions.

Flawlessness in coloured gemstones is even more rare than in diamonds. Gemstones with few or no inclusions are available, but they are usually much more expensive. Even most high-end gemstones are at least slightly included. Being more common, sapphires in jewellery tend to have higher clarity levels than rubies. Internally flawless sapphires are very rare, but internally flawless rubies are extremely rare. When considering a coloured gemstone's clarity, you should measure your expectations against the standard for that variety of gemstone. Some varieties of coloured gemstones, such as aquamarine, blue topaz, and citrine, have naturally fewer inclusions while other gemstones, such as emerald and ruby, tend to have a higher rate of acceptable inclusions.

Type 1 Gemstones that are normally found to be clean.
VVS Very Very Slightly Included. Minute to not detectable - sometimes referred to as loupe clean
VS Very Slightly Included. Minor - sometimes referred to as eye clean
SI1 Slightly Included 1. NOTICEABLE to obvious
SI2 Slightly included 2. OBVIOUS to prominent
I1 Included 1. Prominent – moderate affect on appearance or durability
I2 Included 2. Prominent – severe affect on appearance & durability
I3 Included 3. Prominent – severe affect on beauty, transparency & durability
  Type 1 includes gems such as Amethyst, Aquamarine, Blue Topaz, Citrine, Kunzite, Tanzanite, Yellow Beryl, Yellow Chrysoberyl and Diamond.
Type 2 Gemstones that normally may have a few inclusions.
VVS Very Very Slightly Included. Minor inclusions - sometimes referred to as eye clean
VS Very Slightly Included. Noticeable to Obvious
SI1 Slightly Included 1. NOTICEABLE to OBVIOUS
SI2 Slightly Included 2. OBVIOUS to prominent
I1 Included 1. Prominent – moderate affect on appearance or durability
I2 Included 2. Prominent – severe affect on appearance & durability
I3 Included 3. Prominent – severe affect on beauty, transparency & durability
  Type 2 Colored Gemstones by their nature have natural inclusions. (Inclusions are the norm) This includes gems such as Andalusite, Crysoberyl, Alexandrite, Corundum (i.e. Sapphire and Ruby), Garnet, Peridot, Quartz, Spinel, Tourmaline and Zircon.
Type 3 Gemstones that normally do not come clean.
VVS Very Very Slightly included. Noticeable to obvious
VS Very Slightly included. NOTICEABLE to obvious
SI1 Slightly Included 1. Obvious to prominent
SI2 Slightly Included 2. OBVIOUS to prominent
I1 Included 1. Prominent – considerable affect on appearance or durability
I2 Included 2. Prominent – severe affect on transparency or durability
  Type 3 Colored Gemstones by their nature have many natural inclusions. (Heavily Included is the norm) This includes gems such as Emerald and Rubellite Tourmaline.

Declasse

Stones not transparent because of inclusions.

Cut

When a gemstone is to be used in jewellery, it is cut depending on the size and shape of the rough, as well as the desired piece of jewellery to be made. As a rule of thumb, a cut gemstone will reduce the mass (measured in carats) by at least 50 percent.

There are thousands of different designs for cutting gemstones with the most common shapes being round, oval, square, rectangular and triangular.  Each of these shapes share a set of common sections called table, crown, girdle, pavilion and culet as shown in the diagram to the right.

There is no generally accepted grading system for gemstone cut.  However, like diamonds, a gemstone's cut refers to its proportions and symmetry.  The most usual method of fashioning a gem is to cut the surface into a number of flat faces, known as facets. This gives the stone its final shape and "cut".  Gemstone cut has the greatest impact on the beauty of the stone. The cut impacts how the stone refracts light, how it reflects light and its depth of colour.

Unlike diamonds, not all coloured gemstones have a geometrically ideal cut to maximize brilliance. For gemstones, a high-quality cut exposes the fewest inclusions, displays the majority of the gemstone weight when set in jewellery and presents the most even colour. 

Cut affects the depth of a gemstone’s colour. The deeper the cut, the deeper the colour. In a gemstone with more saturated colour, a shallow cut will permit more light to penetrate, while in a less saturated gem, a deeper cut may increase its vividness.

A well-cut gemstone is symmetrical in all dimensions. It appears balanced and its facets reflect light evenly across the surface, providing good brilliance. It is evaluated by looking at the stone face up and determining the percentage of light returned.  The polish is smooth, without any nicks or scratches.  Facets meet their neighbouring facets at distinct points.

Quality

GemWerx grades the quality of it's cut according to the following:

Quality Description
Master or Precision Cut These gemstones have the highest quality cuts positioned to return the best colour with ideal proportions and symmetry to return maximum brilliance from the gemstone.  Polish is of the highest standard and all facets meet as they should.
Machine Cut Cut by automated or semi-automated machines. These gemstones have the highest quality cuts with ideal proportions and symmetry to return maximum brilliance from the gemstone.  Polish is of a high standard and all facets meet as they should.
Commercial Cut Cutting method unknown or doesn't meet the Master Cut or Machine Cut standard. These gemstones have high quality cuts with good proportions and symmetry to return maximum brilliance from the gemstone.  Polish is of a high standard and most facets meet as they should.
Native Cut This type of cut usually has been cut to return the greatest weight from the rough with the result that the brilliance of the gemstone suffers.   Cutters often use these below standard cuts when they are available in lieu of standard gemstone rough.

Carat Weight

Carat Weight

Weight is an important quality of gemstone value. Often the larger the stone, the more valuable it is, as larger stones are less common. However, it is important to remember that qualities such as colour and saturation are also important in determining a stone's value.

Carat is the unit of measurement used to weigh diamonds and gemstones.  The carat, the standard unit of weight for diamonds and other gemstones, takes its name from the carob seed. Because these small seeds had a fairly uniform weight, early gem traders used them as counterweights in their balance scales.  One carat is equal to 200 milligrams or 0.20 grams (5 carats = 1 gram) and is divided into 100 points; a 10 point gemstone is 0.1 carats.  Gemstones are typically priced on a per carat basis depending on the colour and brilliance of the stone.  And since gemstones rarely occur in large sizes and per-carat prices usually climb steeply as the carat weight goes up.

Generally, as a gemstone's carat weight increases, so does the price per carat. A fun way to look at stone values is the following: since the late 14th century there has been a law for diamonds that is known as the 'Indian Law' or 'Tavernier's Law':

Wt² x C = Price per Stone

The following shows how the price of a sapphire might increase with this formula applied to a $100 a carat base price.

Weight   Total Stone Price
1ct   $100
2ct   $400
3ct   $900
4ct   $1,600
5ct   $2,500
10ct   $10,000

Other

Colour Zoning

Some stones show colours only in parts or layers. To describe the strength of this common but generally unwanted effect, we use four levels:

1.      None: The colour is equally distributed
2.      Faint: One might see changes in colour saturation
3.      Gradual: The colour weakens in some parts but not abruptly.
4.      Visible: Stone has clear colour patches or layers.

Other than clarity, which is judged with a 10x lens, colour-zoning is described only as far as it is visible to the unaided eye.

Colour zoning though in certain gemstones is desirable.  Ametrine, for example, is a mixture of amethyst and citrine and comes in two forms - blended and bicolour - and is a much desired gemstone.