A Background to Pearls

A BACKGROUND TO PEARLS

Pearls come from molluscs, which are a huge family of marine invertebrates, the vast majority of which have shells. The family includes snails, whelks, mussels and oysters. In this article we are concerned only with certain pearl-producing marine oysters and freshwater mussels.

Ropes of Chinese freshwater pearls in natural colours.

All molluscs can produce ‘concretions’, though they vary a lot in appearance. We call the pretty ones ‘pearls’. Some molluscs are better able than others at producing concretions and will do so more readily.

The appearance of pearls varies greatly. Some pearls are porcelaneous (they have a surface resembling porcelain). In this category can be mentioned conch and melo pearls. Other pearls – the most common types in the gem trade -- have a nacreous surface and display a sheen which can be almost iridescent. The quality of this lustre is important in determining the quality of the pearl.

According to one ancient myth pearls are formed when the oysters rise to the surface of the sea and catch dewdrops which, warmed by the rising sun, develop into pearls. Another ancient myth suggests that a pearl is the tear drop of an angel. According to modern myth, a pearl forms when a tiny grain of sand enters an oyster and, unable to expel it again, the oyster coats it with nacre to stop the irritation.

The complete biology behind the formation of pearls is constantly being researched, e.g. whether genetics paly a role, colour, as so forth. We do know, however, what triggers an oyster to produce a pearl and how they grow.

An oyster can expel grains of sand, but if the animal is injured by a larger object becoming lodged inside the shell, or by attack from a parasite, or even a bite from a fish, it may trigger the pearl-producing process. A chain of events is set in motion which causes the oyster to secrete a substance (see below) which forms a pearl. It is this natural process that is harnessed when pearls are cultured.

Left: Black Tahiti pearl (natural colour), and dyed pearl. Right: Two South Sea pearls. All are cultured.

NATURAL AND CULTURED PEARLS

Pearls can be divided into two groups: natural and cultured. Natural pearls occur when an oyster in the wild produces a pearl in response to a naturally occurring stimulation. Cultured pearls occur when a pearl is artificially induced in an oyster.

Natural pearls are today extremely rare and, in many cases, the oysters or mussels which produce them are covered by protection laws, so their harvesting is banned. An example of this is the freshwater pearl mussels found in Scottish rivers.

Cultured pearls can again be divided into two groups: those where a bead has been used as a nucleus to induce a pearl, and those without a bead nucleus. They can also be divided into pearls from marine oysters and those from freshwater mussels. Most freshwater pearls are not bead-nucleated, and pearls from marine oysters have traditionally always had a bead nucleus, but today experimentation is being undertaken on all types and variations.

Left: An assortment of Chinese (non-nucleated) freshwater pearls. Right: Japanese (nucleated) cultured pearls.

THE FORMATION OF A PEARL

It is necessary to know a little about the animal that produces the pearl before we can understand how it performs this task.

Most gem-pearl producing oysters and mussels are bivalves, that is to say they have two hinged shells enclosing a soft body. This consists mainly of the mantle which covers the internal organs and makes the shell. They have no head, a very simple nervous system, and threads called byssus with which they attach themselves to a hard surface. The lining of the shell is covered in mother-of-pearl, which is the same material as the nacre that covers a pearl. We are concerned with the part called the mantle, the outer layer of which (the epithelium) contains glands that secrete the chemicals that make up the shell. If a few cells from the top of this layer are moved into some other part of the oyster’s body, they will continue to secrete these chemicals.

B C

A) Schematic diagram of soft body inside half of shell.

B) Cross section of the area of A marked in red, showing epithelial cells (in red).

C) Pearl sac formed from epithelial cells which have been displaced into the mantle.

For this reason, an injury to the animal that results in some of the cells being pushed, transported, or implanted into another area can result in a pearl, as pearls are made of the same substances as the shell.

When the cells become dislodged from their original situation they form a cyst, with the cells that had formerly been the outside ones (producing the nacreous substance) on the inside. They continue to grow and multiply, so the cyst (the so-called pearl sac), grows larger. Inside the pearl sac a concretion – i.e. a pearl -- is forming.

Shell bead and epithelial cells introduced in a cultured pearl,

In pearl culturing a piece of mantle containing the epithelial cells from a sacrificed oyster is intentionally inserted into a host oyster in order to induce it to produce a pearl. In the case of non-nucleated mussels only the mantle flesh is inserted into a small incision, but with bead-nucleated pearls the incision contains a bead nucleus as well as a piece of mantle tissue. The implanted tissue then forms the pearl sac and secretes nacreous substances. If there is a nucleus inserted, the pearl sack forms around it and will cover it with nacreous substances. In both instances the nacreous substances are deposited continuously in very thin layers.

THE NUCLEUS

The bead nucleus is important and must be of good quality, well formed and polished. Traditionally the bead nucleus used in culturing pearls is made of shell – usually freshwater mussel shell from America – and a whole industry has developed to produce the nuclei. Shell material is the same as pearl material, except in its structure which is in flat layers rather than concentric ones. It is therefore unlikely to be rejected by the host oyster.

Freshwater mussel shell from the Miississippi

basin, which is made into shell bead nuclei.

Another advantage of a shell nucleus is that in the event of the pearl being heated or cooled, there is no risk of the central core reacting differently to the temperature change and loosening or cracking the nacreous covering. Further, it has the same weight as pearl material. Other substances have been used for the nuclei, but with varying success.

PEARL QUALITY

The quality of the pearl depends on many things. According to the age of the epithelial cells, they first produce an organic matrix called conchiolin (a protein related to keratin), then prismatic calcite crystals and finally hexagonal aragonite platelets. Both the last two are calcium carbonate, but in different forms. To have a good lustre, pearls must have layers of aragonite on the surface, which give the optical effects (see below), leading to lustre. The best pearls are produced by epithelial cells of an optimum age. If, for example, the cells are too old, they produce aragonite platelets that are too large, thus diminishing the optical effects and giving less lustre.

The thickness of the nacre in cultured pearls varies from species to species and according to the quality. Although too long in the oyster can be counter-productive, if the bead nucleus is left there for too short time the nacre will be very thin and measure 0.2 mm or even less. Good quality nacre is reckoned to be up to 0.6 mm. The thickness of each individual layer of nacre laid down also affects the lustre, multiple thin layers giving a better quality than fewer, thicker ones.

It is thought that the pearl moves inside the pearl sac. Constant, regular movement may give a very round pearl, but other shapes are also possible. Sometimes the sac is misshapen, or two pearls sacs form close together and combine. In these (and other) situations the result may be one pearl of very irregular shape. If such a pearl still has a good lustre and colour, it may be more highly prized than a round pearl, and called a ‘baroque’.

In the case of bead-nucleated cultured pearls, the round nucleus helps the formation of a round pearl. Non-nucleated pearls can be of very irregular shapes, and indeed it took many years to perfect the round shape of non-nucleated pearls. The first non-nucleated pearls to appear on the market were nicknamed ‘rice crispies’ because of their similarity in shape to the well-known breakfast cereal.

Oysters and mussels can be used more than once to culture pearls. So-called 'cornflake' pearls (again, named after a breakfast cereal the shape of which they resembled), are non-nucleated and formed in the empty pearl sac which collapses after a round bead has been removed.

Left: 'rice crispies', and right: 'cornflake' pearls

(threaded with coral beads and central coral bead).

THE STRUCTURE OF A PEARL

Most commonly pearls are round or almost round. At the centre may be a small amount of conchiolin, which is then followed by layers of prismatic calcite arranged radially. This is followed by tabular crystals of aragonite in a ‘brick wall’ formation. The latter two layers have a very fine matrix of conchiolin surrounding them and giving extra strength.

Left: diagram of formation of concentric layers of nacre around

a shell bead. Right: Concentric layers of nacre as formed in a natural pearl,

or a non-nucleated pearl.

The cause of a pearl’s attractive surface play of colour – known generally as lustre, but also called orient – is complex. It is caused by the action of light rays on the aragonite platelets, where the rays are reflected, refracted and diffracted.

The optical effects of the aragonite crystals are enhanced by their slight irregularity and the unevenness of the surface. Under high magnification a pattern of wavy lines can be seen, and the unevenness gives rise to the very slight roughness experienced when sliding pearls across the teeth – an old and not completely reliable test for ‘real or ‘fake’ pearls which does not of course distinguish between natural and cultured.

In cultured pearls, the bead nucleus is covered by the nacreous substance. Oysters are not machines, but are live animals, so the percentage rate of successful gem-quality pearls being produced is not high. In some cases the cells by-pass the conchiolin and calcite layers, and produce a pearl of good lustre in the shortest possible time. In other cases the bead may be completely rejected. In many cases the pearl is simply not of good quality. A form of ‘mishap’ is when the cells secrete a lot of conchiolin before covering the bead with nacre. This can in fact result in a very attractive grey pearl as the dark conchiolin shows through the white nacre, but the colour is usually uneven and the result very poor.

Left: Good cultured pearl with even layers of nacre. The shell bead is visible

in the drill hole. Right: Broken pearl showing a thick layer of conchiolin

between the nacre and the bead, giving an attractive grey colour.

Poor quality Chinese pearls dyed bright colours.

THE COLOUR OF PEARLS

Pearls occur naturally in colours ranging from silvery white through creams, palest pinks and golden. They also occur in black, though in reality a so-called black pearl will be mid- to dark grey, usually with overtones of purple, green and blue.

Natural pearls get their colour from the oyster in which they grow and are also influenced by outside effects such as minerals in the water. Thus shells with a creamy coloured mother-of-pearl lining are most likely to produce pearls of approximately the same colour.

In the case of cultured pearls the main cause is the origin of the mantle tissue used in the graft when culturing them. This means that it is possible to culture pearls of a different colour to the host oyster. The host oyster can therefore be chosen for its health and ability to accept the graft, while the graft itself can be chosen for its colour and lustre.

PEARL SHAPES AND TYPES

As already stated, most pearls are round, but they can be virtually any shape. They can also be any size, from seed pearls (tiny round pearls) with a diameter of 1mm to the largest baroque pearl (pearl of irregular shape but good quality) of up to 4cm across, though such a pearl is usually at least partially hollow.

Cultured pearls can mean any type of pearl that has been cultivated, but is often used to describe the first type of nucleated cultured pearl, developed in Japan by Mikimoto and his colleagues at the end of the nineteenth century. They are from the Akoya oyster (Pinctada fucata).

P. fucata shell on P. maxima shell.

Three fancy freshwater pears from North American rivers.

South Sea pearls are grown in the Pacific Rim using the very large Pinctada maxima oyster. They range in size from 10 – 18mm and have a beautiful lustre due to the massive quantity of thin layers (about 0.5 microns) of nacre produced by the oyster.

Tahiti pearls is the name given to naturally coloured black cultured pearls from French Polynesia, using Pinctada margaritifera.

Fancy pearls is the name given to pearls of odd shapes. They can grow naturally but can also be cultured using a bead nucleus. Many ‘fancies’ are produced in America using freshwater mussels. The inexpensive ‘coins’ and ‘tablets’ now produced in China are also fancies.

Circle pearls are usually South Sea or freshwater cultured pearls. They have ridges round the entire circumference of the pearl. They vary in vastly in quality and can be found at both ends of the price range.

Keshi is a term often mis-used within the jewellery trade. Keshi was originally used by the Akoya cultured pearl producers to describe very small pearls that where produced accidentally by the oyster during Akoya pearl culturing. It has since been adopted by everyone for any type of tiny pearl, also from freshwater mussels.

'Keshi' pearls in various colours.

Mabé pearls are not true pearls because they form attached to the shell. True mabés derive from just one species of oyster, but today the name is used for all half or blister pearls. They are cultured by attaching a hemisphere of material (resin, shell, etc.) to the inner surface of an oyster’s shell, where the animal’s epithelial cells will cover it with nacre. The resulting blister pearl is then cut out, the shell area removed, and the back of the blister covered with a slice of mother-of-pearl (i.e. nacreous shell). The back is not visible when the blister pearl is mounted in jewellery, for example as ear-rings.

Mabé or blister pearls of varying qualities, and diagram of the cultivation

and production of mabés.

Conch and melo pearls are examples of pearls produced by univalve molluscs, i.e. molluscs with a single shell. They are porcellaneous, and have a typical 'flame pattern' caused by the arrangement of the calcite crystals forming the pearl. They are rare (conch pearls come from the Queen Conch which is today a protected species), and are sensitive to UV light giving them a tendency to fade dramatically. For this reason they have been called 'night-time pearls'. They are not cultivated.

Conch and melo pearls showing

flame pattern.

FINISHING A PEARL FOR SALE

Pearls are finished by cleaning, often including mild bleaching, and they are sometimes polished to remove small blemishes.

They can also be enhanced in many ways, for example by enhancing their colour or changing it completely. Poor quality pearls are often dyed bright colours and sold inexpensively. Pearls can also be copied and imitated, though with the advent of copious amounts of inexpensive fresh water pearls from China, fake pearls are becoming more of a rarity.

PEARL TREATMENTS

Natural pearls are today so rare that they are seldom – if ever – treated. In times past they have been treated to turn them dark grey. This was done by staining them with silver nitrate, which turns black on exposure to light. The same treatment can be given to cultured pearls.

Cultured bead-nucleated pearls from marine oysters can be darkened by irradiation, which affects the freshwater nacre of the bead but not the sea water nacre of the outside of the pearl. The bead nucleus itself may also have been dyed. The result of both methods is a silver-grey pearl.

Brown Tahiti black pearls can be put through various chemical processes to turn them brown.

There are further colour alterations seen on the market today, and some are extremely difficult to distinguish and must therefore be tested for in a professional laboratory.

Any process that alters the colour of a pearl should be declared as the colour can no longer be said to be natural.

TESTING PEARLS

A person who works constantly with pearls will almost always be able to identify the type, and whether it is natural or cultured, the type of culturing used, and whether it has been colour enhanced. For the non-expert it is far more problematic.

Pearls can be tested in gemmological laboratories. The most common test used is probably X-ray, though there are many other tests avaliable

to identify dye and so forth. New equipment and tests have recently been developed but these are available only in a few laboratories.

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Further reading: There are many books on pearls and various aspects of pearl production, their origins and so forth. The most comprehensive to date is:

‘Pearls’, by Elizabeth Strack, which contains clear diagrams, descriptions and photographs.