James Clarke Inc 4Cs

GIA Diamond Color Scale
D E F	Colorless Face up - colorless Face down - colorless
G H I J	Near Colorless Face up - colorless Face down - faint yellow
K L M	Faint Yellow Face up - faint yellow Face down - very light yellow
N O P Q R	Very Light Yellow Face up - very light yellow Face down - light yellow
S T U V W X Y Z	Light Yellow Face up - light yellow Face down - medium yellow
Z+	Fancy
Z++	Fancy Intense
Z+++	FancyVivid
GIA Colored Stone Scale
The second characteristic, tone, is measured between white and black:
0 1 2 3 4 5 6 7 8 9 10	Colorless/ White Extremely light Very light Light Med. Light Medium Med. Dark Dark Very Dark Extremely Dark Black
Saturation
The third characteristic is saturation, measured in six levels from:
1 2 3 4 5 6	Grayish or brownish Slightly grayish or brownish Very slightly grayish or brownish Moderately strong Strong Vivid
GIA Diamond Clarity Grading
In each stone inherent quality parameters, color and clarity, world wide grading usually follows the system established by GIA. The following presents the eleven GIA clarity grades for diamonds.
F1 Flawless	Free from all inclusions & blemishes at 10x
IF Internally Flawless	Minor blemishes only
V VS 1 Very Very Slightly Included	Minute inclusions; Extremely difficult for even an experienced grader to find
V VS2	Minute inclusions; Very difficult for even an experienced grader to find
VS 1 Very Slightly Included	Minor inclusions; Difficult to find
VS 2	Minor inclusions; Somewhat easy to find
SI 1 Slightly Included	Noticeable inclusions; Easy to find
SI 2	Noticeable inclusions; Very easy to find
I 1 Imperfect	Negative effect on appearance and/or durability I 1 moderate effect on either
I 2 Imperfect	Negative effect on appearance and/or durability I 2 - severe effect on either
I 3 Imperfect	Negative effect on appearance and/durability I 3 - severe effect on both factors
Colored Stone Clarity
Type I	Stones normally found with almost no inclusions
Type II	Stones are usually included
Type III	Stones are almost always included

More Info

Stone History
Stone Science
Diamonds
Colored Stones
The 4 "C's"

Carat Weight
Determination of carat weight is quite straightforward if the stone is available un-mounted. The gemologist will place it on a precision scale and measure its weight to accuracy of to 0.001-ct. If the stone is in a setting the weight evaluation is a more complex matter. For stones that are in settings, the gemologist must make many of the dimensional measurements that will later serve to evaluate cut. The weight of the stone will be calculated from formulas developed through study of thousands of stones of all sizes and configurations. One must start with the identification of the stone type or species. Because of their individual composition , some stones are heavier per size than others.

The basic information needed to calculate weights are:

Specific gravity (S.G.)
Shape or cut type of the stone
Length, width and depth (L, W, & D)
Girdle thickness
Bulge factor for step cut pavilions
The formula to calculate carat weight is L x W x D x S.G. x correction factors depending on girdle and bulge.

Calculated weights should be within 10% of the actual weight and are usually within a few percent. Much of the problem with calculating stone weight arises from difficulty or complete inability to obtain measurements. Many times the girdle is completely hidden in a bezel around the stone. If the stone is in a Tiffany or similar mounting, the bottom of the stone is hidden and lack of access to the bottom of the stone and parallax effects the appraiser’s ability to use the table gauge to measure depth. Prongs and bezels will also inhibit the ability to get good length and width measurements.

Color
Color evaluations of most diamonds are performed by different methods and standards than are used for colored stones. Diamonds of fancy colors (yellow, green, blue, purple, etc.) are evaluated as colored stones. To learn why stones have natural color, see information on Gemstone Information and Diamonds.

Most diamonds are graded by standards established by GIA. The GIA grading scale and accompanying color definitions further explain how diamonds are appraised. (see table at left)

Grades 1 – 5 also apply to stones with a grayish hue or a brownish hue.

Fancy diamonds, grades 6 – 8, occur as various shades of red, blue, green, purple, brown, black, and more saturated levels of yellow. Diamonds are color graded under special lighting conditions using daylight equivalent lighting so that the entire visible light spectrum illuminates the stone. They are also examined under ultraviolet light in order to detect level of fluorescence, if any.

The diamonds are placed in a white plastic tray with a dull finish. They are examined from several viewing positions. They are compared against a GIA certified master diamond set of stones which must be well cut and are not allowed to have significant inclusions or any fluorescence which might effect there sole purpose as color masters.

The quality of color is the most important factor in evaluating the quality of colored gemstones. It accounts for about 50% of the overall quality grade. Clarity (25%) and cut (25%) evenly share the balance of the quality evaluation.

Color grading starts with a color wheel, on oval depiction of the rainbow of color hues with red blending into orange, blending into yellow, blending into green, blending into blue, blending into violet, blending into purple, blending into red. These sub- divisions are the hues of the stones.

The second characteristic of color is the tone. Tone is measured between white and black (light and dark) on a gray scale of 0 – 10 with the range of 2 – 8 being most common for gemstones.

The brownish saturation terminology applies to stones in the warm hue range of Purple/Red through Red, Orange, and Yellow to Yellow Green. The remaining hues are considered as cool hues and use the descriptor grayish.

GIA manufactured equipment called the ColorMaster (described earlier) that can generate a vast array of illuminated colors to compare to a subject gemstone. Resultant settings on the instrument when a match is achieved can be converted to a color grade. In addition to the naked eye, charts can be used to evaluate the desirability of the appraised grade for each gemstone species and variety. The GIA ColorMaster is large, extremely heavy and can not be taken on site for evaluation. Although useful, technically it is challenged because the photographs use rear projection of light when the gemstone is illuminated from above. Production on it stopped several years ago as its cost and weight limited the number of purchasers.

An alternate system for color evaluation, also handled by GIA is Gemset. Gemset is a system with about 300 plastic samples of faceted round brilliant shaped comparators to use in evaluating the colored gemstone. These plastic pieces are spaced around the color wheel at selected points and also incorporate tone and saturation variation. The same quality evaluation system as ColorMaster is used. The Gemset is a suitcase sized box when the full set of pieces is purchased. This makes it is cumbersome to carry on site. Experts also challenge the suitability of plastic pieces to simulate gemstones.

Still another color evaluation system is GemDialogue. GemDialogue uses 21 transparencies of various hues with 10 levels of darkness or intensity of each hue for comparison. One may overlay two hues for intermediate readings or add levels of gray or brown to the hue. GemDialogue is a highly portable system and is becoming quite universally applied in the appraisal field as well as among gemstone traders.

Cut
Gemstones are cut in three general categories, cabochon, faceted, and carved. Cabochon cuts are made with curved surfaces. Carvings are artistic representations in stone in either two or three dimensions. Faceted stones are most complex and are composed of many flat, polished surfaces which are arranged to present the beauty of a transparent gemstone by making good use of light.

Although there are many types of faceted gemstone designs, this description will use the round brilliant cut for description. The stone is basically separated into three basic parts, the crown, the girdle, and the pavilion. The crown is the top of the stone. Its primary function is to gather light and focus it down into the stone. The pavilion, at the bottom of the stone, reflects the light that strikes it and keeps it within the stone until it exits back though the crown. The girdle is the band around the stone between the crown and pavilion and it may be faceted or not.

The crown of a round brilliant cut stone has a flat upper octagonally shaped surface called the table. There are triangular facets reaching downward from the eight sides that are called the star facets, since when looked at together with the table, they make an eight pointed star. Between the star facets there are eight kite shaped facets called the bezel facets which reach down to a point at the girdle. The crown is completed with sixteen triangular upper girdle facets in eight pairs, between the lower parts of the bezel facets and the girdle, around the stone. The shape and size of these facets and their angle to the girdle plane are all important to the gemstones optical performance (brilliance and scintillation).

The pavilion has eight main facets that are long kite shaped facets reaching from the point, or the culet, up towards the girdle. Between them and coming down from the girdle are eight pairs of lower girdle facets. At the bottom, there is often a small flat facet called the culet that is parallel to the table and reduces the chance of easily chipping the pointed base. The pavilion is probably the most important aspect of the stone cut as it must be at an angle to the girdle that will keep the light within the critical angle and reflecting within the stone. Shallow or deep pavilions can let light leak out through the bottom on its first or second interface with the stone surface.

Light performance in a stone is evaluated by three factors; windowing, extinction, and brilliance. Windowing is when light leaks out on first interface and you can actually see down through the stone. Extinction is due to leakage from a second interface and creates dark areas when seen from above. Brilliance is when light is returned out the crown. Many colored stones show all of these phenomena, however, the more brilliance, the better.

Stones have two other characteristics which effect optical performance due to cut. Dispersion is an effect where the light is broken up into its rainbow of colors as it leaves the crown facets. Scintillation is the attractive life that is seen with within the stone as it is moved and the light shines and sparkles from the many facets below. People have individual preferences in the cut of the crown as to whether they like a small table that allows more dispersion from larger bezel and girdle facets or a larger table that yields more scintillation.

Clarity
Diamonds: See table (left side of page) for a listing of GIA Diamond Clarity Grading.

Colored Stones: Colored stones are more complex to color grade. Due to the fact that some colored stones occur in nature with few if any inclusions and others are rarely found without several or many inclusions. Colored stones are divided into three types. See table (left side of page).