Nature’s wonder — pearls

Published 28 Nov, 2015 07:05am
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Nature’s wonder — pearls

GEMS, precious stones and metals are mostly mined from Earth. To bring out their beauty, they are cut, cleaned and polished in various ways and a number of times until they attain the perfect shine.

On the contrary, the birth of a pearl is truly a miraculous event — all carried out by Nature itself. They are grown by live oysters far below the surface of the sea and their beauty is such that it needs no further effort to reveal their loveliness. They are born with a shimmering iridescence, lustre and soft inner glow unlike any other gem on earth.

So how is this marvellous stone formed?

Well, almost any shelled mollusc can, by natural processes, produce some kind of ‘pearl’ when an irritating microscopic object (a parasite) or let’s say an intruder enters or slips in between and settles inside the shell of an oyster, a type of mollusc.

Thus, in order to protect itself from irritation and to ease its discomfort, the mollusc quickly begins to cover the uninvited visitor with layers of nacre — the mineral substance (calcium carbonate and conchiolin) that fashions the mollusc’s shells. The process of covering the intruder is repeated many times until the pearl is formed (it takes several years). The pearl is made up of the same luminous, iridescent substance with which an oyster lines the inside of its shell. These are called natural pearls, more valuable than the cultured pearls.

Cultured pearls

SIMILAR to natural, cultured pearls are also formed by the response of the mollusc to a tissue implant (an ‘intruder’) but here the intruder is placed inside the shell by humans. The process involves a tiny piece of mantle tissue (called a graft) from a donor shell is transplanted into a recipient shell, causing a pearl sac to form into which the tissue precipitates calcium carbonate. These pearls are called cultured pearls.

There are a number of methods for producing cultured pearls, like using freshwater or seawater shells, transplanting the graft into the mantle or into the gonad and adding a spherical bead as a nucleus.

Most saltwater cultured pearls are grown with beads; their trade names are, Akoya, white or golden South Sea and black Tahitian. Most beadless cultured pearls are mantle-grown in freshwater shells in China and are called freshwater cultured pearls.

One can distinguish cultured from natural pearls by X-ray examination.

Black pearls (Tahitian)

A NATURAL black pearl is more expensive and mysterious than its classic off-white cousins. And for good reason, although manufacturers can dye pearls black, it takes extremely rare conditions to form pearls that have that dark, eerily iridescent glow.

Natural black pearl is formed in a similar manner when an intruder gets stuck but in a specific type of oyster — the Tahitian black-lipped pinctada margaritifera. The interior shell, called the nacre, of most oysters is usually a glossy white or silver but the ‘Tahitian black-lipped’ oyster features a thick band of black. If the pearl forms near that band, it will suck up that colouring, the closer they are to the band, the darker the colour will be. However, it can also be a silvery grey colour if they get wedged in a lighter portion of the oyster. Tahitian cultured pearls also come in range of greys, blues, greens and browns.

Keshi cultured pearls

KESHI is Japanese for ‘poppy seed.’ Keshi pearls are just accidental by-products of the culturing process. Keshi pearls can form in both freshwater and saltwater molluscs in a couple of ways. ,

The first is when the mollusc expels the bead nucleus but the implanted mantle tissue still remains. This stimulates the pearl sac which instead of coating the now missing round bead nucleus with its nacre, it forms a small keshi pearl instead.

The second way is when the bead nucleus remains inside and is successfully coated with nacre, but implanted mantle fragments create additional pearl sacs this also creates keshi pearls.

Even though these pearls are unintentional, they are still highly profitable for the pearl farmer. Depending on where the keshi are farmed, they can come in all sorts of colours from blues, greys to yellows, white, green and purple.

Conch pearls

LITTLE is known about this natural wonder that cannot be cultivated and is highly sought after. It is naturally created in the shell of the Queen Conch mollusc — a large sea snail found in the Caribbean and South Florida. The rare and elusive conch pearl is prized for its creamy porcelaneous appearance and unique flame effect.

Attempts to culture conch pearls have so far been unsuccessful, which means each and every pearl has been formed naturally. Harvested by teams of fisherman, a single, elusive conch pearl is found in every 10-20,000 shells, although only around one per cent of these are gem quality. This, together with its unusual colour, makes the conch pearl extremely desirable.

Unlike pearls harvested from oysters, conch pearls are not made of nacre, they are formed from calcareous concentration in the mollusc — similar to kidney stones in humans — they have a porcelaneous appearance and a unique shimmer. Oval in shape, they come in a variety of colours, ranging from white, beige, yellow and brown to red and — the most coveted — pink. Many of the highest-quality conch pearls also display a unique flame structure and a weave effect of lighter and darker areas on the surface caused by the formation of aragonite fibres.

Coloured pearls

FRESHWATER cultured pearls vary widely in colour. But some Chinese farmers produce dozens of colours. There are neutrals like white, near neutrals like cream and natural hues like yellow, orange and purple.

To give them colour, farmers immerse these cultured pearls in a bleach solution in large glass jars and place them on wire shelves in small rooms flooded with fluorescent light. The cultured pearls remain in the bleach solution for one to two months, thus they get an unnatural colour.

Melo melo pearls

THESE pearls are non-nacreous and as they are not considered true pearls by gem experts, so they use the term ‘pearl’ in quotation marks when referring to them. These beautiful gem-quality ‘pearls’ come from a gastropod mollusc called a melo melo snail.

This snail inhabits the South China Sea, the waters around the Philippines, the eastern coast of India, and the Andaman Sea. The melo melo ‘pearl’ formation is most likely stimulated by an irritant on the top of its foot. The colour range from light tan to brown but the orange is the most prized colour.

Melo melo ‘pearls’ are usually spherical and quite large. The largest melo melo ‘pearl’ examined weighs 397.52 cts. that is about three quarters the size of a golf ball.

Non-nacreous pearl

SHELLS are made of calcium carbonate which has two distinct, naturally occurring, crystal forms. These are aragonite and calcite. The former tends to be prettier and usually has a more orderly crystal structure, meaning that light reflects through the layers.

While the later, calcite, is the same substance but with a different structure. It does not reflect and refract light like aragonite. It is denser, more like thick porcelain. This calcite makes a non-nacreous pearl!

Mabe (Mah-bay)

THIS is yet another popular category in cultured pearls and are also known as a half pearls. Mabe pearls are unique in their appearance because they are flat from one side while the other side has the bulge. These pearls are grown intentionally by a technician placing a flat bottomed plastic nucleus inside the shell beneath the mantle tissue. The nucleus can be any shape but most often round, oval or heart-shaped.

The oyster then continually deposits nacre on the inside surface of its shell, covering the nucleus and forming a cultured blister pearl. After the nucleus is coated with about 1mm of nacre, a technician harvests the blister pearl and removes the original nucleus, cleans the inside surface and does the procedure to get the perfect cultured mabe pearl.

Did you know

• For many years and also even today, many people believe that natural pearls are created when oysters rise to the surface of water during dawn and collect dew drops in their opened shells, that later turns into pearls. Then from 15th till 17th century, people believed that pearls were nothing but oysters’ eggs. It was only during the 18th century that pearls were known to be created because of the intrusion of a parasite into the oyster.

• Pearls are the only gemstones in the world that are extracted from living animals. While some oysters die after pearl extraction, there are mussels which remain alive even after the removal of pearls.

• Pearls derive their distinctive colours from the inner elements of their shells, which range from white and ivory to pink, black, purple and even gold.

• The oldest pearl jewellery ever discovered was found in the sarcophagus of a Persian princess who died in 520BC. It’s currently on display at the Louvre in Paris.

• In 1916, Jacques Cartier, one of the world’s renowned jewellers was able to purchase his Fifth Avenue store by trading only two pearl necklaces in exchange for that plot.

• There are 17 different types of pearls, falling under the categories of natural, cultured and imitation.

• Natural pearls are the rarest due to over harvesting thus expensive too; even in the early days of pearl hunting, three tonnes of oysters would only produce three or four perfect pearls.

• Pearls come in eight basic shapes: round, semi-round, button, drop, pear, oval, baroque and circled. Perfectly round pearls are the most valuable.