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Facts about Pearls

Pearls have always been popular in India. Today there is a wide range of pearls available: Natural Pearls, Akoya Pearls, South Sea Pearls, Tahitian Pearls, added to that Fresh water pearls and Keshi Pearls. Let us understand each one of them.


Towards the south-eastern tip of Iraq there is a famous harbour town called Basra, well known over the years for natural pearls. Natural pearls are those that have developed inside an oyster without any human intervention. Among natural pearls Basra pearls as well as the pearls from the Persian Gulf command a high price. Most of these pearls are formed in oysters called Pinctada radiate and are entirely of nacreous material which is generally aragonite crystals arranged in a particular manner. Basra pearls indeed have a subtle beautiful sheen and its nacreous lustre has its own charm with body colour white, cream, or cream with iridescent colour sheen. These natural pearls have always been rare and in the present political scenario they will become even rarer.

In the earlier days when the pearls were first extracted from the oyster or mussel, they were tumbled and cleaned to remove residue and odour. Basra pearls are known for being untreated. As cultured pearls are banned in Bahrain and in the Gulf countries only natural pearls are available. Till the time oil was not discovered in the Gulf region, mostly all the smaller pearls (< 3mm) were bought in Doha, Dubai, Bahrain and Catif and sent to India. In Mumbai, they were drilled and made into bunches and were sold as, “Bombay bunches” to the European markets.

Natural pearls in varying sizes are also found in Venezuela, Peru, Ecuador, Panama, CostaRica, Mexico, East Africa, Madagascar and various islands in the Indian Ocean. In India the most important pearl banks were around the coast of Tuticorin and Kayalpattiman. Gulf of Manaar also had pearl fisheries and in Marichchukkaddi in Sri Lanka was a centre for pearl trade. In the pearl trade the Indian unit is “chau” (chov) which actually describes the volume of the pearl. It has a formula with respect to carat weight. Pearls are also sold in India by “tola”.

chau= (carat weight2 x 88) ÷ 135
1 tola= 58.32carats

Bleaching is often used to lighten and even out pearl colour. The first layer deposited over the surface of a nucleus is conchiolin, a dark, porous protein. Bleaching lightens this layer. This is especially important when the nacre is too thin to mask the dark layer, so pearls with thick nacre often do not require bleaching. Bleaching is the oldest method which is applied to Natural Pearls. Japanese factories use a weak solution of hydrogen peroxide. The bleaching process changes the colour pigments contained in the organic substance. The time required is between seven to sixty days, and it is applied until the pearls have reached a uniform white colour.


Pearls cultured in marine oysters (especially in Pinctada fucata) are called Akoya pearls. The famous Mikimoto pearls of Japan are Akoya pearls and the technique of culturing was first started by Mr. Kokichi Mikimoto from Japan. These pearls are cultivated by grafting the oyster with a piece of mantle tissue and a mother-of pearl bead. The nucleating bead is covered by nacreous layer to form a cultured pearl. Size of the bead and the thickness of the nacreous layer (commonly called nacre) determine the quality of the pearl. There are a large number of pearl farms where the oysters are bred nucleated and later pearls are harvested. The freshly harvested pearls are called “hamage”. Pearls are still sold by using the old Chinese-Japanese unit “momme and “kan” is used the wholesale trade. General size of Akoya pearls is between 2.5mm to about 8mm (in rare cases up to 13mm). Cultured pearls with nacreous thickness of more than 0.4mm (to almost 0.6mm) are considered “Gem quality” and those with nacre thickness between 0.3 and 0.2mm are commercial and less than 0.2mm are poor grade.

1momme= 75grains =18.75carats=3.75grams
1kan=1000momme =3.75kilograms

1carat= 0.53momme= 4 grains
1gram= 0.267momme

Akoya pearls are washed and polished by tumbling them in a concentrated water and salt solution that’s abrasive enough to remove any mucus or tissue that clings to the cultured pearls’ surfaces. This is usually followed by chemical treatments that bleach the cultured pearls to make them white.

Pearl farms in Japan

Akoya pearls necklace
Half-drilled Akoya pearls

Most of the Akoya pearls undergo Maeshori treatment. Maeshori is a form of new lustre treatment that involves the heating and cooling of a pearl in order to cause its lustre to be artificially enhanced. It is a Japanese word that literally means before (mae) treatment (shori). Originally, maeshori involved dipping Akoyas in a solvent – mainly methyl alcohol – to clean t hem and remove impurities so they could be bleached more efficiently.

Some Maeshori treatments enhance lustre with solutions of mineral salts, ammonia, and water, or mineral salts and methyl alcohol. One of the Maeshori processes involves soaking pearls in hydrogen peroxide and methyl alcohol. The duration of the pearls being soaked varies anywhere between 30 minutes and 30 days. While in the solution, heat is also usually used to aid the process. Solution treatments work by swelling the nacre platelets, thereby closing or filling minute spaces and creating a tighter, smoother surface that reflects and refracts light better. The result is better lustre.

After this process the pearls are drilled. In the recent times there are many different types of hole-drilling machines available for pearls. Most of these are electrically operated machines.

Pearl drilling machine side view
Pearl drilling machine front view

Medium-to low-quality Akoya pearls (marine water pearls) are often bleached with chemicals after drilling. This whitens them and makes the colour look more even. Improper bleaching can soften the nacre and make it more susceptible to wear, especially if the nacre is thin. High quality pearls do not need to be bleached, and it would be pointless to possibly reduce their lustre and durability by treating them.

According to Fred Ward, Japanese cultured pearl producers (Akoya producers) discovered early on that consumers in different countries preferred specific shades of pearls and so they tinted the pearls. Americans like their pearls with a pink tone, Germans like white pearls, the Indian (especially from west coast), French and South Americans prefer a cream colour, and buyers in Arab nations like them creamier still. "All these pearls are coming out of the same oysters in the same water — something must have been done to produce that range of colours," says Ward. Dyed pearls are typically lower-priced than comparable untreated cultured pearls. If pearls are dyed after drilling, and dye can often be detected by looking at the drill holes and surface imperfections with a loupe for concentrations of colour.


Pearls grown in Pinctada maxima are large in sizes. The oyster size is also large and every oyster can produce only one or two pearls cultured pearls. With the result the prices are also very high. The South Sea pearls are cultured in the islands south of Japan, Philippines, Indonesia, the regions north of Australia, Papua New Guinea, Micronesia, Melanesia and many islands of the Pacific Ocean.

Locations where South Sea Pearls are cultivated

In the South Sea pearls the nuclei used are large 12mm to 18mm and are made from shells of the freshwater mussels. The insertion is along with living mantle tissue. It takes about two to three years for the cultured pearl to develop and is almost 14mm to 22mm in size. Once the pearl is formed it is removed by special long handled spoon and another nucleus is inserted in the same animal for a second harvest. Sometimes even third harvest is also done. The nacre layer of the South Sea pearls is very good it ranges from “1.5mm to 5mm” (average nacre thickness is 2mm to 4mm.

Maeshori treatment:

Harvested pearls are sent for Maeshori treatment. Maeshori treatments vary from factory to factory. It tightens the nacre and pulls moisture out, which will enhance the lustre but also make the nacre more brittle. The procedure expanded to include several other techniques that are also applied to most freshwater cultured pearls and some South Sea cultured pearls. The newer techniques which are designed to enhance lustre, while different, are also called Maeshori and are properly regarded as treatments. This process also sometimes involves adding a coating to the exterior of the pearl. Despite contrary claims South Sea pearls, even those from Australia, are now routinely treated. Untreated pearls will show a softer “creamy” lustre. Japan is Australia’s largest trading partner and regardless of what may happen in the land Down Under as soon as the pearls hit the beach in Japan, they are processed. The procedure known involves chemicals and slight heating. The pearls are heated and then cooled to tighten up the nacre producing a fine lustre.

South Sea Pearls Golden Colour

Iridescent white Colour

Light Cream Coloured

American freshwater pearls, black pearls and light cream coloured South Sea pearls are not normally bleached. However, this is changing with white South Sea pearls. Some of the South Sea Pearls also undergo chemical bleaching. Pearls are dyed to satisfy the tastes of a particular market, to provide colours that are rare, such as golden South Sea pearls, and to make it easier to create a matched strand sometimes dyeing is carried out.

South Sea Treated golden pearl

Untreated South Sea Golden Pearl

Natural coloured golden pearls are usually found in South Sea pearls (Pinctada maxima) and sometimes Akoya pearls (Pinctada fucata) can have a medium yellow colour. It is believed that heat treatment on pearls has been going on for some time. The Chinese have pioneered this method, and are responsible for producing some beautiful golden coloured pearls. Golden pearls are usually a result of natural colour in pearls or by dyeing. Heat treatment is done by placing the pearls for a few days in heating cabinets at a temperature of up to 80°C. This is not a popular colour thus they are bleached or dyed golden yellow. Heat treatment is done to improve the lustre of pearls which is usually done after cold treatment/bleaching (cold treatment means soaking the pearls in bottles of hydrogen peroxide solution under intense light of about 33°C-40°C).

Another treatment enhances lustre by exposing the cultured pearls to hot, dry air in equipment that resembles a fruit or nut dryer. That treatment works by drying and tightening surface nacre. Tighter nacre has fewer tiny spaces, so it achieves the same results as the solution treatment. Neither of these lustre treatments are permanent, and will fade over time. The duration it takes before the lustre fades would vary according to pearl type and the technicians’ skills in these treatments. Pre-treated pearls may fade in about six months and may progress to dullness in a few years. This also means that pre-treated pearls would have a shorter shelf-life.


The black coloured pearls are generally formed as well as cultivated in an oyster called Pinctada margaritifera it is also called the Black lipped pearl oyster.

Untreated Tahitian Black pearls

Rainbow sheen in black pearls

Although black pearls are found in other regions of the globe Tahiti has made its name for black pearls. Tahiti is a small island in the French Polynesia. The Black lipped pearl oysters are large and can weigh up to 5kilograms with sizes up to 30cm. Such large oysters can produce large black pearls. The inside portion of the shell has light grey to silver grey colour with a very dark rim having iridescent colours. When the oysters are about 15cm long they are used for nucleating operation using a nucleus (a bead made from the shell of a mussel) and a piece of the skin (epithelial cells) from another black lipped oyster. It is believed by the Polynesians that the darkest skin patch of the mantle will produce the darker pearls. It takes about 2years to develop into round spherical pearls. During this period the oysters are checked under X-rays to determine whether the pearl is growing properly.


Japanese Freshwater pearls were known in the trade for centuries. It was Dr. Masao Fujita who started the culturing of freshwater pearl in mussels from lakes and sweet water ponds. The cultivation was done by inserting only mantle tissue (no nucleus/bead was inserted) and creating non-nucleated pearls. These were known as Biwako pearls (after the lake Biwa near the old town of Kyoto, Japan).

Chinese Freshwater Mussel

Chinese Fresh water Pearls

North American Freshwater Pearls

Japanese Freshwater Pearls

China, North America and Japan started producing the freshwater pearls. Initially the shapes were like rice grains, later developed into irregular potato shape and finally became complete spherical round shaped freshwater pearls. Freshwater pearls do not have nucleus and are entirely made of nacreous layers with some intermittent layers of protein conchiolin. Each freshwater mussel can develop 50 to 60 pearls inside the shell. Some of these pearls have original pink, salmon, cream colour. The freshwater pearls are dyed artificially into fancy colours as well as black colour to imitate the Tahitian pearls.


In the Japanese language the word “keshi” means something very small like a poppy seed. Generally there are small pearls formed as by-product during the culturing of pearls in Akoya oysters, in South Sea oysters as well as in the Freshwater mussels. Any injury to the animal inside the shell would create a situation for the formation of Keshi pearls. Sometimes the grafted oyster may eject out the nucleus/bead and then the keshi pearl thus formed without the nucleus is much smaller as there is no bead inside. South sea keshi pearls can be large usually in the range 9mm to 12mm. Shapes could be baroque (irregular), egg shaped, drop shaped or any other fancy shape. Colour of the Australian keshi pearl is white and silver gray, Tahitian Keshi pearl are black where as the keshi pearls from Indonesia and the Philippines are creamy white and golden yellow. As there is no nucleus the lustre of these pearls is rather distinctly pronounced.

South Sea Keshi pearls

Petal type Keshi pearls

Akoya Keshi Pearls

Tahitian Keshi pearls

Freshwater Keshi pearls

Indonesian Keshi pearls


Pearls are valued according to their grade. The grading is done generally on the basis of the five factors: size of the pearl, shape, colour of the body of the pearl and the colour of the sheen, lustre (orient) of the pearl with its delicate iridescence and the surface of the pearl. To achieve a certain standard of grade, pearls are also regularly treated and processed. The evaluation of the pearls is carried out on the basis of the type of pearl, the quality grade and the treatments done to enhance the appearance.


Pearls have always fascinated man. Natural pearls may be rare but with the advent of culturing of pearls there is a large variety of different types of pearls. Today there are pearls in different sizes, shapes and colours, among marine water as well as fresh water pearls each with its own enthralling beauty. It is no wonder that pearl is considered the “Queen” among Gems!

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