Grading & evaluation

GIA founder Robert M. Shipley devised the 4Cs (color, clarity, cut and carat weight) in the early 1940s as a memory aid for students. GIA later turned the idea into measured scales. It introduced the D-to-Z color scale in 1953. Earlier systems had used letters, numbers, Roman numerals and terms such as “blue white,” and GIA says it began at D, a letter carrying negative associations, so that its scale could not be confused with theirs. Graders compare each diamond with masterstones, reference diamonds of known grade, under controlled lighting.

Clarity describes inclusions (internal features) and blemishes (surface features). Richard T. Liddicoat and his colleagues set the categories in the 1950s. The scale has eleven grades in six categories, Flawless to I3, judged at 10× magnification by the number, size, relief, nature and location of each feature.

Cut came last. Between 2001 and 2004 GIA gathered more than 70,000 observations of almost 2,300 diamonds, and in 2006 it began issuing cut grades from Excellent to Poor for standard round brilliants. It does not grade the overall proportions of fancy shapes. One carat is 200 mg. ISO 24016:2020 sets terminology and test methods for single unmounted polished natural diamonds above 0.25 ct.

Two finish grades sit beside the cut grade on a GIA diamond report. Polish rates the condition of the facet surfaces; symmetry rates how exactly the outline is shaped and the facets are placed. Both use the words Excellent, Very Good, Good, Fair and Poor, and GIA evaluates them visually at 10× magnification. On round brilliants they feed into the overall cut grade, and GIA accepts either Excellent or Very Good symmetry within an Excellent cut. On fancy shapes, whose proportions GIA does not grade, finish is the main published measure of workmanship.

Fluorescence is the visible glow some diamonds emit under long-wave ultraviolet light. GIA reports its strength as None, Faint, Medium, Strong or Very Strong. About 25–35% of diamonds fluoresce, and more than 95% of those glow blue.

The effect on appearance is disputed. GIA's 1997 study in Gems & Gemology found that most observers saw no relationship between fluorescence and transparency, and GIA puts the hazy exception at fewer than 0.2% of the fluorescent diamonds it receives. Michael Cowing, writing in The Journal of Gemmology in 2010, argued that ultraviolet content in grading lighting lifts the apparent color grade of strongly fluorescent stones. GIA notes that D to H diamonds with very strong fluorescence often sell for less than comparable stones without it.

A diamond whose color lies outside the colorless-to-light-yellow-or-brown range of the D-to-Z scale is a fancy color diamond. GIA grades these stones by a different logic. Graders view the diamond face-up under controlled lighting and describe three attributes: hue, the color itself; tone, its lightness or darkness; and saturation, its depth and strength. GIA recognizes 27 hues, and modifying words record secondary colors, as in “purplish pink” or “orangy yellow.”

Tone and saturation combine into one of nine fancy grades: Faint, Very Light, Light, Fancy Light, Fancy, Fancy Intense, Fancy Vivid, Fancy Dark and Fancy Deep. John King and colleagues published the expanded system in Gems & Gemology in 1994. GIA states that Fancy Intense and Fancy Vivid generally command higher prices.

Color dominates value, and clarity matters less than for colorless stones. Rare hues such as pink, blue and green qualify as fancy colors even in light tones and weak saturation, provided the color shows face-up. Treatment status is a separate service: GIA's Colored Diamond Grading Report carries color and clarity grades, while the origin of the color, natural or treated, appears on its Colored Diamond Identification and Origin Report.

No colored-stone scale has the reach of the diamond 4Cs. Dealers and laboratories judge color first, using the attributes applied to fancy diamonds: hue, tone and saturation, plus how evenly the color is spread. GIA's account of colored gemstone value puts color at about 60% of a gem's worth and treats face-up color zoning, uneven color, as a fault. For ruby, GIA states that pure red to slightly purplish red commands the highest prices.

Clarity is judged against what each species normally shows. GIA notes that some gem materials are almost always included while others are commonly eye-clean. Most faceted aquamarine is eye-clean. Inclusion-free rubies are practically nonexistent. Pink and red tourmaline often carries eye-visible inclusions that dealers accept when the color is strong, while green tourmaline is expected to look clean. Emerald usually shows a visible garden of inclusions, the jardin.

Cut quality is judged by eye. GIA names two common faults: windowing, a washed-out area where light passes straight through a pavilion cut too shallow, and fisheye, a ring formed when the girdle reflects under the table. Symmetry, polish and outline count as well, and cutters weigh them against the weight they would lose.

Some color descriptions carry a price of their own. Pigeon's blood names the finest red ruby and royal blue a deep, saturated sapphire. Cornflower names a softer blue; GIA's origin research ties the velvety look prized in Kashmir stones to dense milky inclusion bands. Padparadscha, from a Sinhalese term for lotus color, names a sapphire between pink and orange.

Each laboratory writes its own definition. GRS says it published pigeon's blood, cornflower blue and royal blue in 1996, and it ties pigeon's blood to a vivid red of low tone, medium to strong ultraviolet fluorescence and a chromium to iron ratio above one. On November 4, 2015, SSEF and the Gübelin Gem Lab agreed shared criteria for pigeon blood red and royal blue: intense, saturated, evenly distributed color, few eye-visible inclusions and no treatment of any kind, heating included. Neither GRS nor the Swiss pair limits the terms to one country. The LMHC defines padparadscha as a subtle mixture of pinkish orange to orangey pink, pastel in tone, low to medium in saturation, seen in standard daylight and from any origin.

The CIBJO Gemstone Book allows a geographic name only where it denotes where the stone was mined. At a CIBJO session in October 2016, GIA's Shane McClure said conflicting laboratory reports arise most often for blue sapphire.

A laboratory opinion that a stone came from a celebrated source can change its price more than most other lines on a report. A landslide exposed the Kashmir deposit in 1881, high above the village of Sumjam in the Paddar valley, and the maharajah's men worked the best of it between about 1882 and 1887, so supply has been fixed ever since. Burmese rubies hold a comparable position: GIA notes that for centuries the Mogok area was practically the only source of marble-hosted ruby.

Auction results show what is at stake, though no two stones are comparable. The Sunrise Ruby, a 25.59 ct Burmese stone, sold at Sotheby's Geneva on May 13, 2015 for US$30.3 million, which GIA records as the first ruby above US$1 million per carat. The 55.22 ct Estrela de Fura, an unheated stone cut from rough found near Montepuez, Mozambique in 2022, sold at Sotheby's New York on June 8, 2023 for US$34.8 million, about US$630,000 per carat.

Premiums do not track quality. GIA's own account of the problem notes that a lower-quality Kashmir sapphire can cost more than a much finer Sri Lankan one, and that deposits inside a single country differ in geology. Buyers pay for the opinion, not for an appearance they could read themselves.

Pearls grow rather than being cut, so they are judged on their own criteria. GIA's 7 Pearl Value Factors are size, shape, color, luster, surface quality, nacre quality and matching. GIA calls luster, the brightness and sharpness of reflections at the surface, perhaps the most important of the seven, and classifies it from Excellent to Poor. Its surface scale runs Clean, Lightly Spotted, Moderately Spotted and Heavily Spotted, and matching again runs Excellent to Poor.

Nacre is the layered material a mollusk lays down around a nucleus or irritant. The CIBJO Pearl Book describes it as microscopic platelets of biomineralized aragonite bound by conchiolin, a scleroprotein. In beaded cultured pearls the nacre must be thick enough for good luster and durability; thin nacre looks dull or chalky and lets the bead show. Color combines the bodycolor with any overtone and with orient, an iridescent shimmer at or just below the surface. Size is given as a diameter in millimeters.

Many sellers use letter scales such as AAA to A. Neither GIA nor the CIBJO Pearl Book defines them. Separating natural from cultured pearls is laboratory work: GIA calls microradiography the most reliable non-destructive way to see a pearl's internal structure.

The word jade covers jadeite, a sodium aluminum pyroxene (NaAlSi₂O₆), and nephrite, a rock of densely interlocked fibres of the tremolite–actinolite amphibole series. GIA now counts green omphacite as a third jade. All are aggregates of tiny interlocking crystals, which makes them exceptionally tough. GIA ranks jadeite on three factors. Color comes first: imperial jade, a vivid green with no hint of gray, sits at the top, and lavender is next. Transparency runs from opaque to semitransparent, and in the best material text read through a thin piece is only slightly blurred. Texture describes grain size as fine, medium or coarse.

Treatment status matters as much as appearance, and the trade uses letter shorthand. A-jade is untreated. B-jade has been bleached and impregnated with resin or wax. C-jade has been dyed, and B+C jade is both. The LMHC recommends that reports state the treatment in plain words, such as “dyed” or “impregnated,” rather than the letters. Laboratories detect fillers and dyes with infrared and Raman spectroscopy and with ultraviolet to near-infrared absorption spectroscopy.