The term natural pearl implies that it is formed accidentally without any human intervention.  

The term natural seawater or oyster pearl implies that the irritant in this pearl is a piece of shell, coral or bone that hooks into the flesh of the oyster. 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.

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A Natural Oyster Pearl occurs when a piece of shell, coral or bone hooks into the flesh of the oyster. 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.  

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A Natural Oyster Blister Pearl can be started by a parasite like a crab that settles on the shell, or a snail, or worm that drills through the oyster shell and dies. These pearls have a flat surface on one side because they have been grown on the inside of the shell.  

A Natural Oyster Seed Pearl is 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.

A Keshi 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 begins secreting nacre layers around itself. 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.  

Natural Freshwater Pearls 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.  

Natural Freshwater Blister have a flat surface on one side of the pearl and are accidentally formed in a pearl bearing mussel.  

Natural Freshwater Seed Pearls are always 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.

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Cultured Oyster Pearls begin when a technician implants into the pearl bearing oyster a nucleus made of mother of pearl with an epithelial cell graft that has come from the mantle tissue of a donor oyster. The graft forms a pearl sac like a placenta; within this sac the oyster secretes layers of nacre to cover the nucleus, eventually forming a pearl. Because these pearls have a nucleus as an irritant they are termed nucleated.

Cultured oyster pearls are normally harvested in the colder months when 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 are kept at bay, each oyster is scrubbed at least once a year and medicinal compounds are applied to the shell.

Out of 1000 cultured oyster hosts:

50% die or eject nucleus.

25% produce pearls of marketable quality.

20% are rejected pearls.

Only 5 % produce top quality pearls.

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Cultured South Sea Pearls are mainly 9-16mm in diameter although they can be well over 20mm; the nucleus is a mother of pearl bead and the nacre 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 host can be as large as 12 inches across and produce pearls that are mainly white in colour with a range of overtones from light gold to pale pink.

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Cultured Tahitian Pearls are mainly 8-18mm in diameter although they can grow to 21mm. The nucleus is a mother of pearl bead. Cultured Tahitian pearls usually have a nacre thickness of 2.5mm; 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 with grey overtones to yellow and pink.

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Cultured Blister Pearls 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.

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Cultured Mabe or 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 .

Mabe pearls are used for earrings and brooches or pins.

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Cultured Akoya Oyster Pearls 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 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 within which the oyster secretes layers of nacre 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 surface, allowing us to see the mother of pearl bead within. Akoya pearls are normally found in all shades of white from very pure white through pale pink to golden tones. They can also be found naturally in pale grey shades, although this is rare. Nowadays many Akoya pearls are treated or permanently dyed to render them a dark colour, a process which leaves the pearls permanently dark.

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A Keshi 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 begins secreting nacre layers around itself. 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.

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Freshwater Cultured Pearls 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 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.

Freshwater pearls are found under the thick mantle of both shells and not in the main body of the mussel; it 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. The mussels wait three years 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 15 pearls each generation; during the harvest all pearls are removed from the pearl sac leaving the sac in place and able to 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.

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Cultured River or Kurasu Pearls. 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.

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Cultured Lake or Sankaku Pearls were first cultured in Lake Biwa, Japan, in the 1930’s. They are also given the generic name of Biwa pearls, and 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 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.

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Cultured Freshwater Seed Pearls are found alongside other freshwater pearls in cultured freshwater mussels and are 2mm or less in diameter.