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Home Gemological Information Diamonds

The 4 C’s of Diamond Grading
GIA Diamond Grading Nomenclature

CARAT WEIGHT

Diamonds are weighed in carats, a term originating from the carob seed, which was used as a unit of weight in ancient times. It takes 5 carats to make one gram. The abbreviation for carat is ct. Carats are divided into 100 units, called points (as in decimal points). Thus, one-half carat equals 50 points, and is written 0.50 ct. One-fourth carat equals 25 points, and is written 0.25 ct., etc.

Because nature produces very few large diamonds, the per-carat price of diamonds increases exponentially with size. For example, a one-half carat diamond would cost much more than two quarter-carat diamonds of the same quality. A one-carat diamond is worth much more than two half-carat diamonds, and very much more than four quarter-carat stones. These price leaps occur around important size divisions, called break points. For example, a 0.99 ct. diamond is priced on a different scale than one weighing a full 1.00-carat.

COLOR

Diamonds are color graded against a set of master diamonds that have been accepted and registered by the GIA or the AGS. Under standardized lighting conditions the diamond is judged by the amount of body color present. The yellow, brown or gray body color begins to be discernible to the naked eye at around the K color grade in mounted gems in the face-up position. Colorless diamonds are very rare, as are naturally colored Fancy Color diamonds. Fancy color diamonds are graded on a scale of their own.

Fluorescence can affect color positively or negatively, depending on its color and strength, and the body color of the stone. Currently, grading lights used by most major laboratories contain enough ultra-violet light to raise the color grade of a fluorescent diamond higher than it will appear under normal light conditions. Furthermore, diamonds with lab certificates stating fluorescence as “None” may actually have fluorescence that would be considered “Faint” or “Medium” by most observers. At our laboratory, we check all fluorescent diamonds over ½ carat both with and without all ultra-violet light filtered out. (See article, The Issue of Diamond Fluorescence.)

Colorless diamonds are very rare, as are naturally colored Fancy Color diamonds. Fancy color diamonds are graded on a scale of their own.

Color exaggerated for illustration 

Diamond_Colors.gif (9617 bytes) 

CLARITY

Clarity is graded on a scale from Flawless to Imperfect. Diamonds below the grade of I-3 are not considered gems, but rather are suitable for industrial uses. Common internal clarity characteristics include: crystalline inclusions, clouds, feathers, pinpoint inclusions, knots and internal graining. External clarity characteristics include: chips, nicks, scratches, bruising, abrasions, knots and surface graining, as well as extra facets and naturals, bits of the original diamond skin purposely retained by the cutter. Eye-visible inclusions or ones that diminish the brilliance or durability of the diamond are observed only in the Imperfect (I-1 to I-3) grades. Fluorescence strong enough to affect clarity is undesirable.

Diamond_Clarity.gif (7887 bytes)

CUT

The GIA, AGS and GCAL laboratories all grade the very important, but highly complicated quality of Cut (proportion, symmetry and polish). Other labs describe cut parameters but do not assign a grade. Premier diamonds, meeting the American “Ideal” standard or one of a number of “Super-Ideal” standards, are comparatively rare and command the highest price. Cut may be further graded for optical symmetry, brilliance and light performance, on a scale of Excellent to Poor. Quality is set by a combination of a number of factors that produce the grade. Similarly, there are parameters used by the trade in evaluating fancy shape diamonds, which have shape, outline and length to width ratio considerations. These parameters vary, according to the shape. The AGS Lab grades the light performance of princess cut diamonds. For more details on diamond cut grading see the GIA Diamond Cut Grading System.

 Round Brilliant Proportion Grading

 

 

EXCELLENT

IDEAL/PREMIER CLASS I

VERY GOOD

FINE –
CLASS - II

GOOD

COMMERCIAL -  CLASS III

FAIR

PROMOTIONAL - CLASS IV

POOR

REJECTION-
UNCLASSED

DEPTH %

57.5%  to 63%

56%  to 64.5%

53.2%  to 66.5%

51.1%  to 70.2%

<51.5%  or  > 70.2%

TABLE

52%  to  62%

50%  to 66%

47%  to 69%

44%  to 72% +

<44%  or  > 72%

CROWN ANGLE

31.5º  to 36.5º

26.5º  to 38.5º

22º  to 40.0º

20.0º  to 41.5º

20.0º  to  > 41.5º

GIRDLE SIZE

Thin to Sl Thick

2.5%  to 4.5%

Ex Thin to Thick

2.0%  to 5.5 %

Ex Thin to V  Thick

0.0%  to 7.5%

Ex Thin to Ex  Thick

0.0%  to 10.5%

Ex Thin to Ex Thick

0.0%  to 10.5%

CROWN HEIGHT

12.5%  to 17.0%

10.5%  to 18.0%

9.0%  to 19.5%

7.0%  to 21.0%

< 7.0%  or  > 21.0%

PAVILION DEPTH

43%

42% to 44%

41%  to 46%

< 40%  or > 46%

< 40%  or  > 46%

STAR FACET

45% to 65%

40%  to 70%

Any

Any

Any

LOWER GIRDLE

70%   to 85%

65%  to 90%

Any

Any

Any

POLISH & SYMM

Excellent to V Good

Excellent to Good

Excellent to Fair

Excellent to Fair

Excellent to Poor

PRICE IMPACT

5%  - 10%   Premium

No adjustment

-15%   to 20%

-20%   to 40%

-40%   to 60%

             


Ideal Cut Diamond

Your gemologist/appraiser can help you evaluate the attributes of a diamond you are considering for purchase, and aid you in making the best selection in terms of your budget and the diamond qualities that are most appealing to you.

If you have read down this far and want to go into diamond grading a little deeper, see the Diamond Knowledge section of Pricescope.com.