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).

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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.