There are many different types of pearl, some rarer than others, before we start differentiating between them lets first understand how pearls are born.
In other words the creation of a pearl is caused by the protective reaction of an oyster or mussel to the accidental or deliberate introduction of a foreign body into its organism. This reaction starts by the mollusc covering the intruder with epithelial cells which will form a pearl sac around the intruder, which in turn deposit concentric layers of nacre that surround the offending object and slowly form the pearl, layer by layer. If the mollusc does not react in this way it will die. To understand further the origins of pearl formation please refer to this link on What makes a pearl... A PEARL
Extract from a speech delivered by Christianne Douglas to the Gemmological Association of London, on the 22nd April 1998.
ON THE EXTERIOR THERE IS NO DIFFERENCE BETWEEN A NATURAL PEARL AND A CULTURED PEARL. The difference arises from the fact that a natural pearl is formed accidentally and without any human intervention, while a cultured pearl is started by the introduction of an irritant by man, after which the process is continued solely by the metabolism of the living mollusc.
A Classification of the different types of pearl which are easily found are listed below with their position in relation to all other pearls.
Occur when a piece of shell, coral, bone or a large piece of grit hooks into the flesh of the oyster, it breaks the surface of the epithelial cells and carries with it epithelial or nacre producing cells. The oyster tries to expel the intruder but if it is unable to dislodge the irritant this foreign body will start the formation of a pearl. A grain of sand is hardly ever involved in the production of a natural pearl as the oyster lives in sand and can easily expel it. Elisabeth Strack an eminent pearl specialist of our day discovered another way in which a natural pearl can be formed, refer to the diagram shown, if the epithelial cells covering of the mantle is broken and these crucial nacre making cells travel as a group into the mantle, this in turn will form a pearl sac and a natural pearl will be born.
The most famous exponent of seawater natural pearls is La Peregrina a drop shape natural pearl which was discovered in the Americas, it was given by Phillip II of Spain to Mary Tudor as a wedding gift. On her death it returned to Spain and was taken to France. Prince Louis Napoleon sold it to the marquis of Abercorn in 1837, the marquis’s son, drilled La Peregrina and recorded its exact weight, 10.192 grams [ over 50 carats in weight]. When, in 1969, Elizabeth Taylor bought a pearl reputed to be the Peregrina for $37,000, its authenticity was challenged. However, due to the exact record of its weight being kept, experts were able to confirm that it was indeed the famous pearl. It was recently sold again by Christies Auction House, for 10 million dollars plus saleroom fees! Goes to show what celebrity status can do for a pearl combined with a market hungry for status.
Can be started by a parasite like a crab that settles in the shell, a worm that drills through the oyster shell and dies, or larva that obstructs a duct within the shell becomes infected and is then covered with the epithelial cells that protect the mollusc. These pearls have a flat surface on one side because they have been grown on the inside of the shell.
Are 2mm or less in diameter. In this category we also find dust pearls which, as the name implies, are tiny and are considered too small for jewellery use.
Is a natural pearl which occurs within a cultured oyster host i.e. in a cultured pearl farm, or when the graft and nucleus in a cultured pearl fail to attach to each other; the oyster rejects the nucleus and the graft tissue forms a pearl sac and secretes nacre layers. The term Keshi in Japanese means seed, these pearls are almost always baroque in shape. Although a Keshi could well be a natural pearl in composition, it must always be described as Keshi, because it grows in a cultured oyster.
The term natural freshwater pearl implies that the freshwater pearl is formed accidentally without any human intervention in a pearl bearing mussel or mulette. Famous natural pearls have come from Scottish, European and American rivers; like the Queen Pearl and the Abernethy Pearl.
Are formed when a small stone or a calcareous concretion lodges in the pearl bearing mussel and starts the formation of a pearl; these pearls have rounded surfaces although they can be of many different shapes. Their colours can be among others white, soft pink, mauve, heather, brown and pale grey.
Have a flat surface on one side of the pearl and are accidentally formed in a pearl bearing mussel.
Are 2mm in diameter or less and have occurred accidentally in a pearl bearing mussel.
The term cultured pearl implies that a technician implants into a pearl bearing mollusc an irritant which must include a graft made from epithelial cells found in the mantle tissue of a donor mollusc. The graft forms a pearl sac and within this sac the mollusc will secrete layers of nacre to cover the irritant. It is important to bear in mind that after the irritant has been introduced the process is continued solely by the metabolism of the living mollusc. The pearl farmer has no control over what these pearl bearing molluscs will produce in size, shape, colour, or even whether they will produce a pearl at all. Each pearl is truly individual.
Begin when a technician implants into the pearl bearing oyster a nucleus made of mother of pearl or resin with an epithelial cell graft that has come from the mantle tissue of a donor oyster. The graft covers the nucleus forming a pearl sac which is rather like a placenta; this sac secretes layers of nacre to cover the nucleus that it is enveloping, layer by layer eventually forming a pearl, the nucleus turns freely within the pearl sac. Because these pearls have a nucleus as an irritant they are termed nucleated. Cultured oyster pearls are normally harvested in the colder months when the nacre layers are thinner and hence the lustre will usually be at its best. Generally each oyster produces only one pearl, which means the oysters are looked after very carefully in a farm; predators like starfish are kept at bay, each oyster is checked and scrubbed at least three times a year.
The following diagram gives an idea of the success rate of oyster pearl cultivation, Out of 1000 cultured oyster hosts:
Are mainly 9-16mm in diameter although they can be well over 20mm; the nucleus is a mother of pearl bead and the nacre can start from 1mm and is usually 2.5mm thick. South Sea pearls are produced in White lipped, Silver lipped and Gold lipped oysters that inhabit the waters of Australia, New Guinea, the Philippines, Indonesia and Burma. The Shell hosts can be as large as 12 inches across and produce pearls whose nacre is coloured according to the mother of pearl coloration of its hosts shell, so white and silver lipped oysters birth pearls that are mainly white in colour with a range of overtones from silver to pale pink, whereas gold lipped oysters will create pearls that go from a light gold hue to a deep orange gold hue.
Are mainly 8-18mm in diameter although they can grow to 21mm. The nucleus is a resin or mother of pearl bead. The nacre thickness of a cultured Tahitian pearl starts at .9mm and usually has a 2.5mm nacre thickness. They grow in the Black lipped oyster which produces black pigment and are found in the surrounding area of the archipelago of French Polynesia, Panama and Mexico. Their colours are naturally dark and range from grey to black and peacock green to aubergine; however, they can sometimes be very light in colour from white to yellow and pink all of which have grey overtones.
Range in size from 4-10mm. These pearls have been cultured on the inside of the shell and consequently have a small flat area where they were attached to the shell. Please refer to the diagram below. Blister pearls are mainly used for earrings and brooches.
Composite Pearls are normally found in sizes from 11mm to 17mm. Mabe pearls are cultivated by attaching a mother of pearl half sphere to the shell itself. Nacre is built up on to this half sphere which is later cut out of the shell, the original dome shaper is removed, the fragile empty half sphere is cleaned with diluted hydrochloric acid, then filled in with resin and a mother of pearl half bead and stopper. The amount of interference by man that goes into strengthening the Mabe pearl gives rise to its other description of Composite pearl. The nacre layers in a Mabe pearl are horizontal as in mother of pearl and not concentric as in a true pearl . See Diagram below, Mabe pearls are used for earrings and brooches or pins.
Range in size from 3mm to 10mm and on rare occasions can be 10.5mm in diameter. Akoya pearls start life when a technician implants a spherical resin or mother of pearl nucleus into the pearl bearing oyster alongside an epithelial cell graft that has come from the mantle tissue of a donor oyster. The graft forms a pearl sac around the resin or mother of pearl implant within which layers of nacre are secreted to cover the nucleus. Akoya pearl bearing oysters must be three years old before they are able to host a pearl successfully and their life expectancy is only seven years. Mikimoto, who was the first to cultivate pearls extensively, recommended that the irritant should be left in the host oyster for three years. At present the accepted time is 1 ½ years, which results in a good 0.5mm coating as seen in the halved pearl pictured on the left. Sadly economic pressures mean that in some cases the irritant is left in the oyster for only six months leaving a nacre coat so thin that the pearl blinks when rolled on a flat white surface, allowing us to see the mother of pearl bead within.
The nacre of Akoya pearl bearing oysters is naturally found in shades of pale bluish grey, hence the pearls from these oysters are normally born in pale blue, deep blue or pale blue/grey colours. Although these colours are rarely found for sale in the market, this is due to the fact that for decades Akoya pearls have been bleached and /or pinked to render them the pale colours that we have grown used to seeing. Bear in mind that the first cultured Akoya pearls were considered “fake natural pearls” and had to imitate as best they could the colours normally found in natural pearls which ranged from white with pink overtones to pale gold pearls. Cultured Akoya pearls nowadays are generally found in all shades of white from very pure white through pale pink to golden tones.
Is a natural pearl which occurs within a cultured oyster host i.e. in a cultured pearl farm, or when the graft and nucleus in a cultured pearl fail to attach to each other, the oyster rejects the nucleus but retains the graft tissue which forms a pearl sac, the graft tissue begins secreting nacre layers to form a pearl. These pearls are almost always baroque in shape. Although a Keshi could well be a natural pearl in composition, it must always be described as Keshi because it grows in a cultured oyster.
They are 2mm in diameter or less and have grown in a cultured pearl bearing oyster.
Are produced in a mussel as opposed to an oyster. The production of nacre is triggered by the insertion of a piece of mantle tissue that contains epithelial cells from a donor mussel; the graft forms a pearl sac which in turn starts the nacre production. These pearls in general have no bead as a nucleus and are therefore termed non-nucleated. When we x-ray a freshwater pearl it shows solid nacre layers and a very small empty cavity at the centre. This cavity originally held the irritant piece of mantle, which has by now dried up, decomposed and disappeared. In recent years some cultured freshwater pearls that have a nucleus have come onto the market, they are termed potato pearls. To form these almost rounds freshwater pearls they farmers have harvested the pearls halfway through their growth term, the harvested pearl is placed in a tumbling machine to render it spherical, then this rounded freshwater pearl nucleus is re-inserted into the mussel which accepts this pearl bead as its own production and proceeds to coat this rounded pearl with more layers of nacre in a almost spherical shape, very akin to a new potato shape, hence their very unflattering commercial name of potato pearl..
Freshwater pearls are found under the thick mantle of both sides of the shell and not in the main body of the mussel; it’s creation is therefore less intrusive to the creature. Some mussels live to be over a hundred years old as in Scotland; the mussels most widely used today for culturing purposes have a life expectancy of 30 years, they await their third birthday before they can host pearl production and are then able to produce up to three generations of pearls before being discarded at the age of 18. Their shells are returned to the water to provide calcium for future generations.
The size of freshwater pearls is purely determined by the amount of time that the pearls spend in the water; the longer they stay in the water the bigger the pearl will become. Each mussel can produce up to 10 to 40 pearls each generation; during the harvest all pearls are removed from the pearl sac leaving the sac in place and able to heal itself and produce future generations. The main producer of cultured freshwater pearls is China, as it has the technology to produce and improve pearls and labour costs are very low.
These pearls, cultured in the Kurasu mussel, are shaped like a grain of rice with a surface of wrinkled appearance. They were extensively cultured in China and flooded the market in the 1980’s. Otherwise known as Rice Crispy pearls, they were normally found in white or cream shades, but at one point they were dyed in bright, sometimes almost fluorescent, colours.
Also known as BIWA PEARLS were first cultured in Lake Biwa, Japan, in the 1930’s. Their size depends on how long the pearl has been growing in the water. The characteristic smooth skin and deep lustre of Biwa pearls makes them instantly appealing to the eye. They come in a variety of rich natural colours that range from white through cream to apricot, from heather or mauve to pale pink and from brown/copper to pale grey. Biwa pearls come in a variety of shapes as the irritant is a piece of tissue that can be cut into several different forms; they range from long strips, to round flat beads, from oval pearls to fantastic shapes that look like crosses, dogs and mythical dragons. Recently it has been possible to find almost perfectly round spheres; it is also possible to find these pearls in a variety of dark shades, having been treated or dyed in such a way that the colour is permanent.
Are found alongside other freshwater pearls in cultured freshwater mussels and are 2mm or less in diameter.
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