Pearls are graded on seven value factors: size, shape, color, luster, surface quality, nacre quality and matching. The Gemological Institute of America (GIA) uses this seven-factor framework, and most dealers describe pearls in similar terms. There is no single worldwide grade scale the way there is for diamonds, so a letter grade from one seller may not match the same letter from another.
Size
Pearls are measured in millimeters across their diameter, or by length and width for off-round shapes. Strands are described by the range, such as 7.0 to 7.5 mm. Size depends heavily on the type of pearl, because each mollusk can only carry a bead of a certain size.
| Pearl type | Common size range |
|---|---|
| Akoya | About 2 to 10 mm, most often 6 to 8 mm |
| Freshwater (tissue nucleated) | About 2 to 12 mm, larger for bead-nucleated |
| Tahitian | About 8 to 15 mm, occasionally larger |
| South Sea | About 9 to 16 mm, occasionally larger |
Within one type, price rises quickly with size, since large pearls need more time in the water and more of them fail. A 9 mm Akoya is far scarcer than a 7 mm one. Natural pearls are sometimes weighed in grains, an old unit equal to a quarter of a carat. See natural pearls for more on that trade.
Shape
Shape falls into broad groups:
- Round: perfectly spherical, the rarest and usually most expensive shape.
- Near-round: slightly flattened or elongated, visible only on close inspection.
- Oval and button: oval is longer in one direction, button is flattened like a squashed sphere and suits earrings.
- Drop: shaped like a teardrop, prized for pendants and earrings when symmetrical.
- Semi-baroque: noticeably irregular but still somewhat symmetrical, including drops and ovals with small bumps.
- Baroque: freely irregular forms. Freshwater baroques can be large and highly lustrous.
- Circled: any shape with one or more grooves or rings running around it, common in Tahitian and South Sea pearls.
Symmetrical shapes generally cost more than irregular ones of the same quality. Baroque pearls have their own following, and Renaissance jewelers often built pendants around them. The page on Renaissance jewelry shows how they were used.
Color and overtone
Pearl color is described in three parts. Body color is the main color, such as white, cream, gold, gray or black. Overtone is a second, translucent color seen over the body color, often rose, silver or green. Orient is the shimmer of iridescent colors on or just below the surface of some fine nacreous pearls. The page on what pearls are made of explains how the layered nacre produces these effects.
Preferred colors depend on the type and on market taste. White Akoya pearls with a rose overtone are a classic. Deep golden South Sea pearls and Tahitian pearls with a green-purple peacock overtone are also in demand. Color does not change durability, so graders treat it as a matter of preference and rarity. Any dyeing should be disclosed. See gemstone treatments for how pearl treatments fit into wider disclosure norms.
Luster
Luster is the brightness and sharpness of light reflected from the surface. Many gemologists treat it as the most important single factor, since even a large, round pearl looks lifeless without it. Graders roll pearls on a neutral background under even light and look at the reflection of the light source.
- Excellent: reflections are bright and sharp.
- Very good: reflections are bright and near sharp.
- Good: reflections are bright but not sharp, slightly hazy at the edges.
- Fair: reflections are weak and blurred.
- Poor: reflections are dim and diffused, giving a chalky look.
Luster depends on how smooth and well-stacked the nacre layers are. Thin nacre and fast growth tend to give weaker luster.
Surface quality
Surface quality covers blemishes such as spots, bumps, pits, wrinkles, abrasions and chips. Graders consider how many there are, how large and how visible, and whether any could weaken the pearl. Typical categories run from clean (few or no visible marks) through lightly blemished and moderately blemished to heavily blemished. A completely flawless surface is very rare. A buyer stringing a necklace can often hide a small mark near the drill hole.
Nacre quality
For bead-nucleated cultured pearls, nacre thickness matters for both looks and durability. A thin coat can let the bead show through as a stripe or blink when the pearl rolls, and it can crack or wear through near the drill hole. Graders treat nacre as acceptable when the bead is not noticeable and the pearl has no chalky look. A bead that shows through, or a dull, chalky surface, signals thin nacre. Labs can measure nacre thickness with X-ray imaging.
Tissue-nucleated freshwater pearls are solid nacre, so this factor matters less for them. The cultured pearls page explains how bead and tissue methods differ.
Matching
Matching applies to strands, earrings and any piece with more than one pearl. Graders check how uniform the pearls are in size, shape, color, luster and surface. A well-matched strand looks even from end to end, or graduates smoothly from a larger center pearl toward the clasp. Assembling a closely matched strand takes sorting through many pearls, so matching adds real value.
Letter grades: A to AAA
Many sellers grade Akoya and freshwater pearls on an A, AA, AAA scale, sometimes with AA+ or AAAA added. AAA is presented as top quality, with high luster, a nearly clean surface and good shape. A is the lowest commercial grade. The scale has no official owner, so each company sets its own thresholds. Compare actual luster, surface and shape, along with photos or videos, before trusting the letters.
Tahitian A to D
French Polynesia historically used an A to D system for Tahitian pearls, focused mainly on surface quality and luster. Under the commonly cited thresholds, A pearls have very high luster and marks on no more than about a tenth of the surface. B pearls have high or medium luster and marks on up to about 30 percent. C pearls have medium luster and marks on up to about 60 percent. D pearls have marks spread over more of the surface, but none deep. Pearls below D, along with those with too thin nacre, were barred from export under government rules. The Tahitian pearl farming page covers that regulation history.
Putting the factors together
Every factor trades against the others. A smaller pearl with excellent luster and a clean surface can cost more than a larger one with weak luster. For jewelry worn every day, luster and nacre thickness matter most for long-term appearance. For investment-level pieces, labs and reports matter most. The guide on how to buy a gemstone covers return policies and documentation, and colored gemstone grading shows how other gems face the same lack of a single standard.
Frequently asked questions
What is the most important factor in pearl quality?
Luster is often treated as the most important, because it controls how bright and alive a pearl looks. Size and shape have the largest effect on price within a type.
Is AAA always the best pearl grade?
AAA is usually a seller's top grade, but the scale is not standardized. One company's AAA can be another's AA, so judge the pearls themselves.
Do baroque pearls have lower quality?
Baroque pearls usually cost less than round pearls of similar luster and surface. Shape is only one factor, and a lustrous baroque pearl can be a fine pearl.
How is nacre thickness checked?
Labs measure nacre thickness with X-ray imaging. At home, watch for a stripe or blinking effect as the pearl rolls, or a visible bead boundary at the drill hole.