The chemistry of a pearl

Seven processes, and the maison's choice.

On a pearl, after the harvest, seven processes can be carried out. They lighten, colour, brighten the surface, fill, correct. They serve those who start from pearls of modest quality and need to intervene to arrive at a beautiful result.

MIMÍ begins much earlier, and by another road. The maison buys only the finest pearls: the selection takes place at the places of origin, specimen by specimen, and only the top three to five per cent of each harvest reaches the MIMÍ standard. The chosen gems arrive at the jewel in the condition in which they were born. The seven processes therefore remain outside the maison's work.

One thing remains to be said, and it concerns every pearl, even the most precious. On its surface, small natural characteristics can almost always be observed. A pearl is organic matter, grown inside a living mollusc, and those marks are the proof that it is a genuine pearl. This page describes the seven processes, one by one, and states the maison's position on each.

The seven processes the maison leaves outside its work

These are the interventions the pearl market carries out after the harvest. MIMÍ avoids them by choice: the maison wants pearls of natural appearance, and to have them it selects only the finest.

Bleaching. A bath of hydrogen peroxide makes the pearl lighter. It is used on gems whose colour is uneven or darker than desired, to bring a whole lot to a uniform hue.
The maison leaves this process aside: it selects pearls whose light colour is native.

Dyeing. Eosin deposits a rosy film on the surface; silver nitrate takes the pearl to grey and black. The resulting colour takes the place of the native one, and it is applied to gems whose natural hue has little commercial value.
The maison leaves this process aside: the colour of a MIMÍ pearl is the one produced by the mollusc.

Irradiation. Gamma rays emitted by cobalt-60 shift the colour towards a deep blue. It is used above all on freshwater pearls, to obtain dark tones that remain rare in nature.
The maison leaves this process aside: the dark hues of MIMÍ pearls are native.

Heat treatment. A hot bath shifts the colour centres of the crystal lattice, and cream becomes gold. It concerns light-coloured pearls, brought in this way to a hue that is worth more on the market.
The maison leaves this process aside: the gold of a MIMÍ pearl descends from the inner rim of the shell that produced it.

Oiling. A vegetable oil brightens the surface and evens out its appearance. The effect lasts a few weeks, then fades: it serves to present gems of modest lustre at their best.
The maison leaves this process aside: the lustre and orient of a MIMÍ pearl belong to its structure.

Peeling. A blade removes layers of nacre where the surface carries a conspicuous characteristic. It makes the pearl smooth the same day, and reduces its thickness for ever.
The maison leaves this process aside: a MIMÍ pearl keeps whole the nacre the mollusc deposited.

Filling. A drop of glue, introduced through the drill holes, reattaches the nacre to the nucleus from which it has come loose. It is applied to gems that have lost the adhesion between the pearly layer and the nucleus.
The maison leaves this process aside: in a MIMÍ pearl the nacre adheres to the nucleus as it formed.

Gemmology lists and describes these processes precisely, and the trade makes use of them. Whoever buys a pearl jewel has the right to know which of them have touched their gem.

This is the maison's declaration.

What we declare	What it means
Native colour	The hue and the overtone are those the mollusc produced. The maison selects exclusively cultured pearls of natural colour, preserved in their original condition.
Whole nacre	Every layer the mollusc deposited stays on the pearl, in the thickness in which it grew: the maison's pearls keep their original nacre.
Original bond	The nacre adheres to its nucleus as it formed, free of any filling.
Light of its own	Lustre is the sharp reflection of the surface; orient is the iridescence that surfaces from beneath, produced by light passing through the layers. In a MIMÍ pearl both belong to the structure of the gem.
Hand-knotted on silk	Akoya, South Sea and Tahitian strands are knotted by hand, pearl after pearl, on silk thread.

Among the seven, peeling deserves a further word, since it touches the life of the gem. The thickness of the nacre governs the lustre and defends the pearl from daily wear: a thinned pearl gains in smoothness the same day and loses for ever the matter that protects it. A pearl that keeps its natural characteristics also keeps all the nacre the mollusc deposited.

Public-domain images: Rijksmuseum, Amsterdam, and Wellcome Collection, London.

Among Akoya pearls, native grey and gold belong to very few specimens: when the selection comes across them, the maison declares it.

Why a MIMÍ pearl is worth more

A pearl that comes beautiful from the harvest is rare, and its rarity has a price. That is the pearl the maison buys. The comparison is made beside the MIMÍ master pearls, and the gem is accepted only when lustre, orient, shape, surface and nacre quality correspond. Every pearl then passes through three checks — at the producer, on arrival in Italy and on the finished jewel — and is measured with precision callipers to 0.01 mm. The value of a MIMÍ pearl lies in what reaches the jewel intact: the colour the mollusc produced, all the nacre it deposited, and the small natural characteristics that speak of its authenticity.

The seven factors by which a pearl is read, and the difference between lustre and orient, are found in How a pearl is judged. What determines the value of a gem: What determines the price of a pearl.

Text by the MIMÍ Milano gemmologists · updated 9 September 2026