How a pearl is judged

Size, shape, colour, lustre, surface, nacre quality and the harmony of the matching: a guide to the seven factors by which a pearl is read.

A pearl forms in the dark, inside a mollusc, over a span of years. A foreign body reaches the animal's vital parts, and the animal responds by enveloping it. It deposits calcium carbonate in minute crystals and, between one layer and the next, a web of conchiolin that holds them together. It repeats the operation thousands of times. The material that results is called nacre — in Italian perlagione, and also mother-of-pearl — and each of its layers records the conditions in which the pearl grew.

The maison's pearls are cultured: the human hand delivers the initial nucleus to the mollusc, and the animal completes the rest of the work in the years that follow. The exceptions are the conch and the melo, natural gems born by pure accident, to which a separate page is devoted.

Pliny the Elder, in the Naturalis Historia, offered another explanation: oysters rose to the surface to drink the dew, and the pearl depended on the purity of that drop. Clear rain, white pearl; turbid sea, flawed pearl. Science has corrected the mechanism and confirmed the intuition: a pearl records the conditions in which it formed, and whoever knows how to read it finds them there.

To judge a pearl, seven factors are observed, the same ones the GIA recognises as value factors. Six concern the single gem: size, shape, colour, lustre, surface and nacre quality. The seventh, the harmony of the matching, concerns the way several pearls sit together in a strand or in a pair of earrings. What follows explains each factor: what it is, how it is measured, what to look for.

The seven qualities

01 · Size

It is measured in millimetres, with precision callipers: to the whole or the half millimetre for Japanese pearls, to the second decimal for South Sea pearls, where a tenth shifts the value. Large sizes remain rare for a physiological reason: a large nucleus strains the mollusc, which more often returns it to the sea.

The orders of magnitude. Akoya pearls range from 2 to 10 mm, and the most common sizes lie between 7 and 8. Freshwater pearls range from a few millimetres up to 20 mm and beyond. South Sea pearls start at 8 mm and reach 16, up to 22 for Australian whites. Tahitian pearls usually lie between 8 and 13 mm, and few exceed 16. Between a 7 mm pearl and a 9 mm pearl of the same quality the difference in price is sharp, because the growing time changes.

In the maison, sizes are measured with precision callipers, with a resolution of 0.01 mm.

02 · Shape

Gemmology recognises seven shapes: round, three-quarter, near-round, drop, button, circled, baroque. The perfect sphere is rare: the mollusc deposits the material around the nucleus following its own movement, and that movement is seldom regular.

What to look for. In a strand, shape is judged on the whole: pearls of similar shape, matched with patience, are worth more than the same pearls gathered at random. Shape weighs on value as much as size, and at times more.

In the maison, the shape of every pearl is checked individually against the declared type and the MIMÍ master pearls.

03 · Colour

The colour of a pearl is read in two stages. Along the circumference one observes the body colour: white, cream, pink, purple, gold, grey, black. At the centre, where the light gathers, one observes the overtone, which the Italian trade calls sfumatura and which may shift towards rosé, green or silver. The observation is made from above, on a white background, in diffuse northern light. Under these conditions the same pearl, viewed by different people, receives the same judgement.

To the body colour and the overtone a third element is added, the orient: the iridescence that surfaces from beneath, the play of colours that moves across the pearl as it is tilted. It arises from the exceedingly thin layers of aragonite. Light passes through them, is sent back by each of them and splits apart, as happens on the film of a soap bubble. In gemmological classification the orient belongs to colour, alongside body colour and overtone, and is to be kept distinct from lustre: lustre is the light the surface sends back, orient is the light that returns from within.

What to look for. Colour depends on the mollusc that produced the pearl, and the rarer a hue the more it is worth: among Akoya pearls, native grey and gold belong to very few specimens; among South Sea pearls, the hue descends from the inner rim of the shell; among Tahitian pearls, the iridescent green the trade calls peacock comes from Pinctada margaritifera alone.

04 · Lustre

Light striking a pearl penetrates some way into the layers before turning back. Along the way it is reflected, refracted, scattered, and interferes with itself. The light that emerges has a depth of its own, and that depth depends on the layers it has passed through. Lustre tells how much of that light the surface returns and how sharply: it is the measure of how far the pearl behaves like a mirror. Orient, described above, is the light that rises from within. The two properties are distinct, they grow together and are judged together.

What to look for. On a pearl of intense lustre the reflection of a window stays small and sharp-edged, and the pearl seems lit from within. Where the lustre is weak, that reflection widens and fades. Lustre depends on the order in which the crystals are arranged and on the thickness of the layers the light passes through: lustre and nacre quality therefore depend on each other.

In the maison, lustre is assessed through the brightness and sharpness of the reflection, by direct comparison with the MIMÍ master pearls.

05 · Surface

Gemmology lists precisely the characteristics a surface may carry — scratches, cracks, pits, small bumps, hammered aspect, protuberances, spotting, banding — and observes them with the naked eye, under diffuse light, turning the gem. Each formed during the growth of a layer and stays where it is, like a ring in the trunk of a tree.

What to look for. A pearl is made of thousands of layers, and almost always carries some surface characteristic: it is the sign of organic matter, grown inside a living mollusc. What counts is the number, the depth and the position: a mark on the side that rests against the skin weighs less than one at the centre of the pearl.

In the maison, only natural characteristics compatible with the declared MIMÍ grade and with the overall beauty of the jewel are admitted.

06 · Nacre quality

At the centre of almost every cultured pearl lies a small sphere of mother-of-pearl: it is the nucleus that the human hand delivered to the mollusc. Around that nucleus the animal has worked for years, and what it has deposited is the nacre. Its quality is measured by the thickness and compactness of the layers. To the eye that thickness stays invisible: fresh from the water, a pearl with thin nacre and one with generous nacre have the same surface.

What tells them apart is time. Nacre determines three things: the lustre, because it is the thickness the light passes through; how the surface withstands daily wear; and the condition of the pearl thirty years from now. Thin nacre wears away with rubbing, and the nucleus begins to show through — the flaw the trade calls a peeling pearl.

How it is checked. Against the light, through the drill hole, the layers can be told apart; in the laboratory, X-ray measures their thickness while leaving the gem intact. It is the only one of the seven factors that stays silent to the eye.

In the maison, every pearl is accepted only when nacre quality, lustre and structural integrity correspond to the MIMÍ master pearls. For Akoya, South Sea and Tahitian pearls, integrity and compactness of the nacre are required, together with a clear surface in which the nucleus stays invisible.

07 · Harmony of the matching

The first six factors concern the single gem. The seventh arises when pearls are brought together: in a strand, in a pair of earrings, in a composition of several rows. The harmony of the matching measures how closely the pearls correspond to one another in colour, overtone, orient, shape, size and lustre, and how regular the sequence appears to the eye that travels along it.

What to look for. A strand is observed from end to end, in natural light, turning it: pearls that resemble one another give a continuous sequence, those that differ interrupt it. Where the sizes are graduated, the scale must rise and fall regularly towards the central pearl. It is the factor that takes the most time: to compose a homogeneous strand of large, luminous pearls, thousands of specimens must be selected, and the price takes full account of that work.

In the maison, the matching is composed by hand, gem by gem, and checked a second time on the finished jewel.

Before the measuring comes the choosing

The maison buys directly from the producers, at the places of harvest, before the pearls leave for the great international centres. The selection takes place there, specimen by specimen, and from that moment begins the journey that leads to the finished jewel.

The MIMÍ Pearl Standard

The seven factors become a judgement through the MIMÍ Pearl Standard, the method by which the maison accompanies every pearl from selection at origin to the check on the finished jewel. The method weighs the seven value factors recognised by the GIA and translates them into the proprietary MIMÍ scale: AAA, AA and A+. Every MIMÍ pearl reaches at least grade A+, and the average quality selected is AA.

The grade is assigned by comparison: the pearl is observed beside the MIMÍ master pearls, the reference specimens the maison keeps for each type. Every pearl passes through three checks — at the producer, on receipt in Italy and on the finished jewel — and is measured with precision instruments to 0.01 mm. Final approval rests with MIMÍ's internal quality control.

The scale has three grades and rises upward: A+, AA, AAA, with AAA the highest grade. It sets no laboratory thresholds: it says how closely a pearl comes to the master pearls in the factors by which the grade is assigned — lustre, orient, surface, nacre quality, shape, colour and the harmony of the matching. The judgement is made by eye, batch by batch, with the pearl set beside the reference specimen, under constant light and background. It is a comparison, and for that reason it is repeatable: the specimens stay the same over time and the conditions of observation do not change.

A note on the GIA. The reference to the Gemological Institute of America concerns the seven value factors and the terminology by which they are described. The AAA, AA and A+ scale is proprietary to MIMÍ and is applied by the maison through the experience of its own gemmologists and comparison with the master pearls. A certificate issued by a third-party gemmological laboratory — among them GIA and Gübelin — accompanies exceptional pearls only.

The four pearls compared

Sizes are measured with precision callipers, with a resolution of 0.01 mm.

The maison's four pearls each have a page of their own: Akoya, South Sea, Tahiti, Freshwater. The baroque shape runs across all four types and has a page of its own: today the maison offers a wide selection in Freshwater. What the maison declares about the integrity of the gem is set out in The chemistry of a pearl; how to keep a pearl over the years, in How to care for pearls.

How these seven factors translate into the value of a jewel: What determines the price of a pearl.

Text by the MIMÍ Milano gemmologists · updated 9 September 2026