- A pearl is a hard object produced within the soft tissue (specifically the mantle) of a living shelled mollusc.
- Just like the shell of a mollusc, a pearl is made up of calcium carbonate in minute crystalline form which has been deposited in concentric layers.
- The ideal pearl is perfectly round and smooth, but many other shapes of pearl occur. The finest quality natural pearls have been highly valued as gemstones and objects of beauty for many centuries, and because of this, the word pearl has become a metaphor for something very rare, fine, admirable and valuable.
- The most valuable pearls occur spontaneously in the wild, but they are extremely rare.
- Cultured or farmed pearls from oysters make up the majority of those that are currently sold.
- Pearls from the sea are valued more highly than freshwater pearls.
- However, almost all species of shelled molluscs are capable of producing pearls of lesser shine or spherical shape.
- The English word pearl originated (via French perle) from the Latin word perla. Margaret means pearl in Greek (the English name Margaret originated from the Greek word for pearl).
- Almost any shelled mollusc can, by natural processes, produce some kind of “pearl” when an irritating microscopic object becomes trapped within the molluscs mantle folds, but the great majority of these “pearls” are not valued as gemstones. Nacreous pearls, the best known and most commericially significant pearls, are primarily produced by two groups of molluscan bivalves or clams. A nacreous pearl is made from layers of nacre, by the same living process as is used in the secretion of the mother of pearl which lines the shell.
- A “natural pearl” or “wild pearl” is one that forms without human intervention at all, in the wild, and is very rare. Many hundreds of pearl oysters or pearl mussels have to be gathered and opened, and thus killed, in order to find even one wild pearl, and for many centuries that was the only way pearls were obtained. This was the main reason why pearls fetched such extraordinary prices in the past. A cultured pearl is formed in a pearl farm, using human intervention as well as natural processes.
- One family of nacreous pearl bivalves – the pearl oyster – lives in the sea, while the other – a very different group of bivalves – lives in freshwater; these are the river mussels such as the freshwater pearl mussel. Saltwater pearls can grow in several species of marine pearl oysters in the family Pteriidae. Freshwater pearls can grow within certain (but by no means all) species of freshwater mussels in the order Unionida, the families Unionida and Margaritiferidae.
- The unique lustre of pearls depends upon on the reflection, refraction, and diffraction of light from the translucent layers. The thinner and more numerous the layers in the pearl, the finer the lustre. The iridescence that pearls display is caused by the overlapping successive layers which break up the light falling on the surface. In addition, pearls (especially freshwater cultured pearls) can be dyed yellow, green, blue, brown, pink, purple or black.
FRESHWATER & SALTWATER PEARLS
- Shell of one species of freshwater pearl mussel is called Margaritifera margaritifera.
- Freshwater and saltwater pearls may sometimes look quite similar, but they come from different sources.
- Natural freshwater pearls form in various species of freshwater mussels, family Unionidae, which live in lakes , rivers, ponds and other bodies of fresh water. These freshwater pearl mussels occur not only in hotter climates, but also in colder more temperate areas such as Scotland. However, most freshwater cultures pearls sold today come from China.
- Saltwater pearls grown within pearl oysters, family Pteriidae, which live in oceans. Saltwater pearl oysters are usually cultivated in protected lagoons or volcanic atolls.
CREATION OF A PEARL
- The difference between wild and cultured pearls focuses on whether the pearl was created spontaneously by nature – without human intervention – or with human aid. Pearls are formed inside the shell of certain molluscs as a defence mechanism against a potentially threatening irritant such as a parasite inside its shell, or an attack from outside, injuring the mantle tissue. The mollusc creates a pearl sac to seal off the irritation.
- The mantle of the mollusc deposits layers of calcium carbonate (CaCO3) in the form of the mineral aragonite or a mixture of aragonite and calcite (polymorphs with the same chemical formula, but different crystal structure) held together by an organic horn-like compound called conchiolin. The combination of aragonite and conchiolin is called nacre, which makes up mother-of-pearl.
- The commonly held belief that a grain of sand acts as an irritant is in fact rarely the case. Typical stimuli include organic material, parasites and even damage that displaced mantle tissue to another part of the molluscs’ body. These small particles or organisms gain entry when the shell valves are open for feeding or respiration. In cultured pearls, the irritant is typically an introduced piece of the mantle epithelium, together or without a spherical bead (beaded or beadless cultured pearls).
- Natural pearls are nearly 100% calcium carbonate and conchiolin. It is thought that natural pearls form under a set of accidental conditions when a microscopic intruder or parasite enters the bivalve mollusc, and settles inside the shell. The mollusc, being irritated by the intruder, forms a pearl sac of external mantle tissue cells and secretes the calcium carbonate and conchiolin to cover the irritant. The secretion process is repeated many times, thus producing a pearl. Natural pearls come in many shapes, with perfectly round ones being comparatively rare.
- Typically, the build up of a natural pearl consists of a brown central zone formed by columnar calcium carbonate (usually calcite, sometimes columnar aragonite) and a yellowish to white outer zone consisting of nacre (tabular aragonite). The presence of columnar calcium carbonate rich in organic material indicates juvenile mantle tissue that formed during the early stages of pearl development.
- Displaced living cells with a well defined task may continue to perform their function in their new location, often resulting in a cyst. Such displacement may occur via an injury. The fragile rim of the shell is exposed and is prone to damage and injury. Crabs, other predators and parasites such as worm larvae may produce traumatic attacks and cause injuries in which some external mantle tissue cells are disconnected from their layer. Embedded in the conjunctive tissue of the mantle, these cells may survive and form a small pocket in which they continue to secrete their natural product: calcium carbonate. The pocket is called the pearl sac, and grows with time by cell division; in this way the pearl grows also. The juvenile mantle tissue cells, according to their stage of growth, produce columnar calcium carbonate, which is secreted from the inner surface of the pearl sac. With ongoing time the external mantle cells of the pearl sac proceed to the formation of tabular aragonite. When the transition to nacre secretion occurs, the brown pebble becomes covered with a nacreous coating. As this process progresses, the shell itself grows, and the pearl sac seems to travel into the shell. However it actually stays in its original relative position within the mantle tissue. After a couple of years, a pearl will have formed and the shell might be found by a lucky pearl fisher.
- Cultured pearls are the response of the shell on a tissue implant. A tiny piece of mantle tissue of the donor shell is transplanted into a recipient shell. This graft will form a pearl sac and the tissue will precipitate calcium carbonate into this pocket. There are a number of options for producing cultured pearls: use freshwater or seawater shells, transplant the graft into the mantle or the gonad, add a spherical bead or do it non-beaded. The large majority of saltwater cultured pearls are grown with beads, the trade name of the cultured pearls are Akoya, white or golden South Sea, or black Tahiti. The majority of beads-less cultured pearls are mantle grown in freshwater shells, trade name, Chinese cultured pearls.
- Cultured pearls (bead-less or beaded) and imitation pearls can be distinguished from natural pearls by X-ray examination. Nucleated cultured pearls are often “pre-formed” as they tend to follow the shape of the of the implanted shell bead nucleus. Once the pre-formed beads are inserted into the oyster, it secretes a few layers of nacre around the outside surface of the implant before it is removed aster six months or more.
- When a cultured pearl is X-rayed, it reveals a different structure to that of a natural pearl. A beaded cultured pearl shows a solid centre with no concentric growth rings, whereas a natural pearl shows a series of concentric growth rings. A bead-less cultured pearl (whether of freshwater or saltwater origin) may show growth rings, but also a complex central cavity, witness of the first precipitation of the young pearl sac.
- A well equipped gem testing laboratory is able to distinguish natural pearls from cultured pearls by using a gemological x-ray in order to examine the centre of a pearl. With an x-ray it is possible to see the growth rings of the pearl, where the layers of calcium carbonate are separated by thin layers of conchiolin. The differentiation of natural pearls from non-beaded cultured pearls can be very difficult without the use of this x-ray technique.
- Natural and cultured pearls can be distinguished from imitation pearls using a microscope. Another method of testing for imitations is to rub two pearls against each other. Imitation pearls are completely smooth, but natural and cultured pearls are composed of nacre platelets, making both feel slightly gritty.
VALUE OF A NATURAL PEARL
- Quality natural pearls are very rare jewels. The actual value of a pearl is determined in the same way as it would if it were any other precious gem. The valuation factors include size, shape, and quality of surface, orient and lustre.
- Single natural pearls are often sold as collectors items, or set as centre pieces in unique jewellery. Very few matched strands of natural pearls exist, and those that do often sell for hundreds of thousands of dollars. (In 1917, jeweller Pierre Cartier purchased the Fifth Avenue mansion that is now the New York Cartier store for US$100 cash and a double strand of matched natural pearls valued at the time at US$1 million).
- Keshi pearls, although they often occur by chance, are not considered natural pearls. They are a by-product of the culturing process, and hence do not happen with out human intervention. These pearls are often quite small: typically a few millimetres in size. Keshi pearls are produced by many different types of marine mollusc and freshwater mollusc in China. Today, many Keshi pearls are actually intentional, with post harvest shells returned to the water to regenerate a pearl in the existing pearl sac.
ORIGIN OF A NATURAL PEARL
- Previously, natural pearls were found in many parts of the world. Present day natural pearling is confined mostly to seas off Bahrain. Australia also has one of the worlds last remaining fleets of pearl diving ships. Australian pearl divers dive for south sea pearl oysters to be used in the cultured south sea pearl industry. The catch of pearl oysters is similar to the numbers of oysters taken during the natural pearl days. Hence significant numbers of natural pearls are still found in the Australian Indian Ocean waters from wild oysters. X-ray examination is required to positively verify natural pearls found today.