Jewelry Appraisal - Diamond Grading

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.

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.)

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.

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.

	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%

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.