Natural Pearl

Natural Pearls are organic products formed inside a pearl oyster due to the concentric deposition of nacreous layers around any foreign body which has entered the animal accidentally, without the aid of any human agency.

How are natural pearls formed?

This is the result of an accident.

• A fish searching for food passes near an oyster and finding nothing moves off elsewhere by using its tail.
• This movement causes a cloud of sand, from which several grains lodge in the oyster.
• The oyster can get rid of most of these, except those which bury themselves in its flesh.
• These, along with a few epithelial cells will be changed into pearls.
• In a pearl, only aragonite is formed, in concentric layers.
• The aragonite crystals have their optic axes at right angles to their main surfaces, so that all the optic axes in a pearl are radial.
• This is important not only in the creation of the pearl's lustre and orient but in determining the differences between a natural and a cultured pearl.
• The aragonite layers are not continuous, but have irregular edges where a layer is not complete.
• This is the cause of the roughness of a natural or cultured pearl when rubbed against the teeth, compared with the smoothness of an imitation pearl.
• Depending on the parasites which attack them, and the way in which they react, various pearl formations are produced.
• Among these are found Baroque pearls, Blister pearls and Concretions.
• Salt water and fresh water oysters and mussels can produce nucleated and non nucleated pearls.
• Blister Pearls are swellings which are formed on the inner surface of the oyster shell or mussel.
• They must be removed very carefully from the shell so that the layer of mother-of-pearl remains.
• In some cases, when cut from the shell, the base remains flat and without a pearly coating.
• If they were detached, the pearl might well be too small or too baroque in its shape.
• Baroque Pearls are formed within the mantle and are distorted and irregular in shape. They occasionally resemble familiar objects such as teeth, tadpoles, mushrooms or snails.
• Pearl Concretions are caused by the amalgamation of a number of small natural pearls in the mantle of the oyster. Pearl concretions are always baroque in shape and rarely of much value.

The chemical composition of a pearl can vary according to its place of origin. In certain cases the quantity of organic material or of water percentage is different. But these variations are very minor and the following figures are fairly accurate.

Calcium Carbonate (Aragonite)	91.72%
Organic Material (Conchiolin)	5.94%
Water	2.23%
Other Substances	0.11%

Colour: Pearls are found in many colours. Colour in pearls is defined by:

• Body colour is the predominant basic colour of the pearl.
• Overtone is the one or more colours that overlie the body colour.
• Orient is the play of colour effect observed over the surface. It may be a combination of pink, blue, green and silver shades.

Cause of Colour: The main factors which are responsible for the colour in pearls can be classified with respect to the environment and the chemical and optical properties.

• Environment
  1. Presence of mineral salts and salinity in the water. This is related to the rock types and the associated soil e.g.
     • A small amount of magnesium carbonate, without iron or aluminum gives the white colour.
     • Iron gives a light brown colour.
     • Copper carbonate (azurite) gives a blue colour.
     • Zinc carbonate (smithsonite) gives a grey colour.
     • Cream and gold contain a large percentage of magnesium carbonate, iron oxide and aluminum.
  2. Amount of plankton. Water rich in plankton gives the pearl a light green tint.
  3. Temperature of the water. In general, moderate temperature produces finer lustre in pearls, warmer waters produces duller lustre.
• Chemical and Optical properties: Chemicals include the presence of chromophore (colour causing elements), while the optical effects include the quality of internal reflections and interference of light. As a result colours obtained are pink, white, silver (optical) and yellow, salmon pink, black, blue (chemical).

Natural Pearl Identification

The separation of natural pearl from cultured pearl is not possible without the use of certain distinct testing techniques. With regular improvements in the culturing process it is getting more and more difficult to conclusively identify natural pearls.
On the whole, the percentage of natural pearls available is very minimal in comparison to the cultured pearl availability. Most of the following are indicative tests and not conclusive. But when performed together, they provide conclusive evidence.

1. Tooth Test: Rub the pearls lightly along the surface of your upper teeth.
   • If they feel gritty or rough, they may be pearl.
   • Care should be taken since it may scratch the pearls if done improperly.
   • Both natural and cultured pearls often exhibit the same reaction.
   • As such this test can be used to distinguish between Imitation pearls and Natural / cultured pearls.
2. Drill Hole Test: Examine the drill hole area with a magnifying lens.
   • If a dark dividing line separating the nacre from a pearl bead nucleus is visible, the pearl is a cultured pearl.
   • If a series of growth lines, which get more yellow or brown towards the centre, is seen, it is a strong indication of a natural pearl.
   • The diameter of a drill hole can also give some detail about the nature of the pearl.
   • In the case of a cultured pearl, the drill hole measures 0.06mm while in a natural pearl it is rarely larger than 0.04mm.
3. Lucidoscope: This technique uses the fact that the transparency of the layers of nacre (mother-of-pearl) varies depending on the direction from which it is viewed under a strong beam of light.
   • This can be observed with the naked eye or with a lens.
   • A cultured pearl with an MOP nucleus would be more transparent along the parallel layers of the bead nucleus than down the length.
   • This effect is better observed in immersion rather than in air.
   • Commonly pure benzene is used since it evaporates quickly and therefore does not harm the pearl.
   • This technique fails when thick skinned cultured pearls are examined.
4. Pearl Compass: The principle of the instrument is that a certain position according to its crystallographic magnetic field.
   • This instrument consists of powerful magnets, in the centre of which is suspended the pearl to be tested using a torsion free thread.
   • Due to differences in their internal structures, a cultured pearl with a mother-of-pearl core will cause the pearl to turn because of its layered structure.
   • The radial and concentric structure of a natural pearl does not exhibit the turning movement.
5. Endoscope: This instrument consists of a powerful source of light within housing. The light beam is then passed through a fine hollow needle which has an aperture and two polished mirrors at its end aligned in opposite directions at 45° to the beam of light. The light beam travels through the tube and reflects from the first inner mirror through the aperture into the pearl being examined.
   • In the case of a natural pearl, this reflected light enters the concentric layer structure and travels round it by a process of total reflection, until it strikes the second mirror.
   • It is then reflected off this mirror and out through the drill hole where it will be seen as a bright flashes when examined through the microscope.
   • In the case of a cultured pearl due to the layered structure of the nucleus, the light is not reflected onto the second mirror, but instead is transmitted through the parallel layers and is lost to the observer and no flash of light is seen.
6. Lauegrams: A narrow beam of x-rays passed through pearl exhibits a pattern of spots on a photographic plate.
   • The pearl must be examined in three different directions to obtain an accurate result.
   • Two distinct patterns are observed for natural and cultured pearls.
   • Natural pearls will exhibit a hexagonal spot pattern in all directions.
   • Cultured pearls will exhibit a hexagonal and a tetragonal spot pattern depending on the direction in which they are examined.
7. X-Radiography: This test relates to the transparency of the pearl structure to X-rays.
   • Conchiolin, which is an integral part of the pearl structure, is more opaque to X-rays than calcium carbonate.
   • As a result, a natural pearl will show a concentric structure from the centre to the surface.
   • In a cultured pearl either the central portion is broad (non-nucleated) or with parallel layers surrounded by concentric structures (nucleated).
   • A complete string of pearls can be examined at a single exposure.
8. Fluorescence: In general, natural pearls exposed to X-rays do not exhibit any fluorescence. However, Australian pearls and freshwater pearls have been known to fluoresce. Cultured pearls generally tend to fluoresce under X-rays with the exception of some non-nucleated pearls.

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Cultured Pearl

Cultured pearls are nacreous formations secreted in the interior of productive molluscs. The secretion is started by human intervention and the rest of the process is performed by the molluscs themselves. Cultured pearls were first licensed in A.D. 1907. The names of Mise, Nishikawa and Mikimoto are almost always linked with the history of cultured pearl production. Pearl culturing these days is a highly organized, commercial enterprise which is very quality conscious.

How are pearls cultivated?

Pearl culturing requires the following criteria to be fulfilled to obtain a good yield and quality.

1. Pearl Farm: Factors to be considered are:
   • Temperature of the water should be fairly constant, with little fluctuations.
   • The salinity and the mineral salts must be considered.
   • The type of sea-bed, whether rocky or sandy, and its accessibility.
   • The presence of mild currents is important as they provide the food and oxygen to the oysters.
   • The farming region should be protected from natural climatic hazards.
2. Oyster preparation and requirements:
   • The oysters are starved for several days to ensure that they can be opened much more easily, their gonads emptied and that the risk of rejection of the nucleus is reduced.
   • Cores which are used as the nucleus are made from the shells of freshwater mussels (mainly those from the Mississippi). Their specific gravity is similar to that of a pearl and they have a neutral white colour. The shell is cut into thin sheets, then cubes, which are rounded in a tumbler. Diameters can vary as per the demand.
   • Grafts: A graft is a small piece of the mantle which contains the cells producing the mother-of-pearl or nacre. This is inserted along with the nucleus and is responsible for the formation of the pearl. A strip is cut from the edge of the mantle and is carefully cleaned. The size of each graft should be such that it covers about one third of the core. A graft can live for about two hours in contact with seawater and should be utilized almost immediately.
   • Operating Theatre: Since the entire process is similar to a major operation, care should be taken to maintain hygienic conditions and well trained staff so as to control and prevent the death of oysters.
3. Culturing Process: This involves the following stages and can vary slightly for the production of round pearls and mabe pearls.
   • The oyster is carefully opened with a sharp knife and held open by placing a wooden chock between the shells.
   • The gonad or the reproductive system is the organ in which the implant of the graft and core is best tolerated by the oyster and produces the best results. This produces round pearls.
   • The graft and the core are inserted into the gonads to the required position.
   • Mabe pearls are produced by inserting a dome - shaped nucleus of mother of pearl, between the mantle and the shell of the oyster.
   • During a single operation, two cores can be implanted at the same time.
   • The oysters are then returned to the sea for a period of two to three years or until sufficient nacreous coatings have been deposited.
   • The pearl is then extracted from the pearl sac. In some cases, another core is inserted and the oyster returned to the sea.
   • In the case of mabe pearls, it is cut from the shell, cleaned and the hollow hemisphere is filled with a polyester resin and then closed with a mother of pearl backing.
   • Harvesting is done during the winter season when the layers deposited are finer and with a better lustre. Once the harvest is over the pearls are processed to remove any organic impurities. The pearls are slightly bleached, which on drying gives a whiter appearance without the green tinge that freshly collected pearls always have.
   • The pearls are now sorted on the basis of shape, colour, orient and size.
     • Shape plays a major role in determining the price of pearls. Pearls can be generally considered under the following categories:
       • Round
       • Seed pearls (very small rounds)
       • Slightly flattened round
       • Baroque (irregular, pear shaped etc.)
     • Colour is defined by the body colour, overtone and orient, e.g. a white body colour with a pink overtone known as rose, or a silvery or white overtone, a natural black or steel gray body colour with good orient is fairly highly priced.
     • Orient should be judged using a standard daylight lamp against a dead white background.
     • Size is measured in terms of weight and diameter, e.g.
       • 1 carat = 4 pearl grains
       • 1 momme = 18.75 carats
     • Drilling of pearls: A pearl may be half drilled or completely drilled as per the requirement. Drilling is commonly done using a hand drill.

Types of Cultured Pearls

In trading circles a number of different terms are used to define pearls from specific locations or those of a particular type. The following are a few of the terms.

• Akoya Pearl: Saltwater pearls from the Akoya Oyster (pinctada fucata martensii). These are usually cultured, round, and their natural colours are pink, white, and yellowish.
• South Sea Pearls: This refers to large white pearls cultured in the pinctada maxima oyster - a large oyster found in the South Seas. There are two types of oyster called the silver-lip or gold-lip depending on the colour of the lip of the shell. Commonly found in the salt water area extending from Myanmar and Indonesia, down to Australia and across to French Polynesia.
• Black Pearl: These are grey to black coloured cultured pearls. Cultivated in the South Pacific from the black-lip oyster, pinctada margaritifera. The colour is an inherent characteristic of the pearl nacre. These are naturally black and not those that have been dyed.
• Biwa Pearl: These are freshwater cultured pearls cultivated in Lake Biwa, Japan's largest lake. Nucleated and non-nucleated pearls are cultured here.
• Mabe Pearl: In the manufacture of a mabe a half-bead, dome-shaped nucleus (often of plastic or soap stone) is stuck on the inner surface of the shell. Once the nacre is secreted over the bead, the blister pearl is cleaned to prevent deterioration. The hollow shaped base is then filled with a paste, wax or resin and covered with a mother-of-pearl backing. The term blister mabe is also used.
• Mikimoto Pearl: Pearls produced and marketed by the Mikimoto Co. They are known for having a higher lustre and fewer flaws than the average Akoya pearl.
• Half Pearl: A whole pearl that has been ground or sawed on one side, usually to remove blemishes. If about 3/4 of the pearl remains it is called a half pearl. The term half pearl is also used to refer to blister pearls.
• Blister Pearl: Natural or cultured pearls that grow attached to the inner surface of the oyster or mussel shell, and when cut from the shell, one side is left flat with no pearly coating.
• Keshi Pearl: naturally occurring very small, round / baroque, non-nucleated pearls, which form accidentally as a by - product during pearl formation (natural or cultured). Keshi is the Japanese word for poppy seed.

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Pearl

Meaning: Pearls (Indian: Moti; Italian: Perla) are classified into two basic categories - Natural and Cultured. The term synthetic must not be used with respect to pearls. Materials which simulate or look like pearls, but do not have either the composition or structure, are known as imitation pearls.

Information on Pearls

Pearls are produce by molluscs which are saltwater oysters and freshwater mussels.
The oyster consists of a soft visceral mass enclosed between two halves of the shell which are hinged together. Like every other animal, an oyster possesses a heart, stomach and mouth. It breathes through its gills and feeds itself with micro-organisms and planktons in the water.

• Pearl Oysters belong to the Pinctada family of the mollusc group in class Lamellibranchia. Pinctada molluscs are saltwater oysters. Molluscs like Unio and Hyriopsis are freshwater mussels.
• The shell is composed of various layers.
• The first layer is dark horny outside layer composed of organic substance conchiolin, a protein like substance.
• It is also the binding agent that holds the aragonite crystals together.
• The inner layer is a thicker crystalline layer of prismatic calcium carbonate in the form of calcite.
• Mother of Pearl is the smooth pearly lining (layer) on the interior of a mollusc shell. It is composed of CaCO₃ as aragonite crystals and a little water and conchiolin.
• Aragonite layer is made up of microscopic platy calcium carbonate crystals arranged in an overlapping manner.
• Nacre is a protective pearly substance secreted by the mollusc and deposited around an irritant, in layers. These layers decide the lustre and quality of a pearl. The thinner the layer, the better is the lustre and vice versa.
• Mantle is a fold of epithelial material that envelopes the internal organs of the animal and is situated at the meeting point of the inner sides of the shell. It secretes conchiolin, aragonite and calcite.
• Orient of a pearl results from the diffraction of the light through the aragonite crystals.
• When white light is reflected from two different surfaces very close together, the two reflected waves can be out of phase, which causes interference, resulting in rainbow colours.
• The finer the pearly layers, the greater the orient that the pearl will possess.

 Pearl Source:

• Natural Pearl
  • Saltwater Pearl
    • Persian Gulf
    • Australia
    • Japan
    • Straits of Manaar (Region between India & Sri Lanka)
    • South Sea Islands
  • Freshwater Pearl
    • Mississippi (U.S.A.)
    • China
• Cultured Pearl
  • Saltwater Pearl
    • Japan
    • Australia
    • Philippines
    • South Sea Islands
  • Freshwater Pearl
    • Lake Biwa (Japan)
    • China
• Imitation Pearl are products, completely or partially man-made, imitating the appearance, colour and effect of natural or cultured pearls without possessing the physical and chemical properties, even when using natural substances.

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