Sunday, May 24, 2009

The GIA type Color Scale:

The 3 charts below are based on the GIA (Gemological Institute of America) "Color Stone Grading System", where the Color with it's various Hues, the Tone, and the Saturation of color in a particular gem is listed.
You will normally see this system identified by a letter or set of letters, followed by two numbers. The letters are abbreviations of the Color and Hues visible... the first number is the Tone or lightness/darkness of a particular gemstone... and the second number in sequence is the Saturation of color in the gem. So, as an example, when you see a Ruby given the color "R", this would indicate the color is "Red" meaning the primary color is Red and there appears to be no noticeable secondary color... if the tone/saturation numbers were listed as"6/5"... this would indicate the gem has "Medium-dark" tone and "Strong" saturation of color... making this particular gem fall in the "good" category.

GIA Type COLOR SCALE

Abbreviation
HUE
Abbreviation
HUE
P
Purple
styG
strongly yellowish-Green
rP
reddish-Purple
yG
yellowish-Green
RP/PR
Red-Purple or Purple-Red
slyG
slightly yellowish-Green
stpR
strongly purplish-Red
G
Green
slpR
slightly purplish-Red
vslbG
very slightly bluish-Green
R
Red
vstbG
very strongly bluish-Green
oR
orangy Red
GB/BG
Green-Blue or Blue-Green
RO/OR
Red-Orange or Orange-Red
vstgB
very strongly greenish-Blue
rO
reddish-Orange
gB
greenish-Blue
O
Orange
vslgB
very slightly greenish-Blue
yO
yellowish-Orange
B
Blue
oY
orangy-Yellow
vB
violetish Blue
Y
Yellow
bV
bluish Violet
gY
greenish-Yellow
V
Violet
YG/GY
Yellow-Green or Green-Yellow
bP
bluish Purple


The Tone and Saturation Scale:
Is based on a determination of the lightness or darkness of a particular gemstone.
Tone should be considered along with the Color to properly understand the depth of color in the gem. Even though it may have good primary color, if it's too light in tone, it will not be rich enough... or... if the color is too dark, it will sacrifice brilliancy and transparency.
Saturation should be considered along with the Color and Tone to properly understand the amount and evenness of the color that is saturated throughout the gem.

GIA Type TONE SCALE

0
1
2
3
4
Colorless or White
Extremely Light
Very Light
Light
Medium-Light





5
6
7
8
9
Medium
Medium-Dark
Dark
Very Dark
Extremely Dark
GIA Type SATURATION SCALE
1
2
3
4
5
6
Brownish or Grayish
slightly Brownish or slightly Grayish
very slightly Brownish or very slightly Grayish
Moderately Strong
Strong
Vivid


Understanding the GIA type Colored Stone Grading System :
Here's how a gemologist evaluates a gemstone's hue, tone, and saturation, using the GIA type colored stone grading system :

The stone is first identified as to gem type. After being thoroughly cleaned, the stone is held by its girdle over a neutrally colored background, and the gemologist looks at it face-up from a comfortable distance – usually about 18 in. (45 cm) – in strong, diffused, daylight-equivalent lighting, with the light source somewhere around 10 in. (25 cm) above the stone. The stone is then rocked back and forth up to a total of 30 degrees as the tone, hue, and saturation judgments are made.

The following examples will help you understand the process involved in how a gem’s color is determined. Let's first look at the illustration to the right. This example tone chart starts with #2 very light, and goes to #8 very dark, as the tone examples. The gem’s tone is determined first, and in this example it is determined that the tone is #5 medium.

Next, the gemologist estimates the overall hue. Look at the illustration to the left to see which hue compares best to the examples on the hue wheel. The sample stone is green, with a touch of blue, so the gem is very slightly bluish green. The hue shows up as bright flashes of brilliance when you look at the stone in the face-up position. In a stone with a window that occupies over 50 percent of the face-up area, the gemologist also grades the window as the dominant color and then the brilliance as the additional color.

Lastly, the gemologist estimates the saturation level. Look at the illustration to the right. Saturation is how much color the gem has. If it does not appear to be grayish or brownish at all, it has strong to vivid saturation. Saturation is also where any additional colors such as those due to color change, pleochroism, color zoning, and windowing less than 50% are noted. Saturation has a moderate to strong effect on appearance. Additional colors, like a window as noted above, are graded only if they are moderate to strong.

The sample stone is thereby graded very slightly bluish green (vslbG), medium tone (5), vivid saturation (6).


The COLORLESS GEM Color Scale:
.
The chart below is based on the Diamond Color Grading System.
COLOR GRADING of Diamonds arbitrarily begins with "D". "A", "B", and "C" are assumed to not exist.

COLORLESS GEM GRADING SCALES

COLOR GIA* CIBJO*
COLORLESS D EXCEPTIONAL WHITE+
E EXCEPTIONAL WHITE
F RARE WHITE+
NEAR COLORLESS G RARE WHITE
H WHITE
I VERY SLIGHTLY TINTED WHITE
J SLIGHTLY TINTED WHITE
FAINT YELLOW
to
FAINT BROWN
K TINTED WHITE
L
M TINTED COLOR 1
VERY LIGHT YELLOW
to
VERY LIGHT BROWN
N TINTED COLOR 2
O
P
Q
R
LIGHT YELLOW
to
LIGHT BROWN
S TINTED COLOR 3
T
U
V
W
X
Y
Z
CHAMPAGNE
CONAC
GREENISH
BROWNISH
C+
FANCY COLOR
YELLOWISH
PINKISH
BLUISH
LIGHT FANCY
YELLOW
GREEN
BLUE
PURPLE
PINK
RED
FANCY
FANCY INTENSE
VIVID
* GIA = Gemological Institute of America.
* CIBJO = Confederation Internationale de la Bijouterie, Joaillsrie, Ortevrerie, des diamants, perles et pierres precieuses.

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