All About Pearls

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Pearl Index:

Natural Pearls

Pearls have been prized for their beauty and rarity for more than four thousand years. From ancient China, India, and Egypt, to Imperial Rome, to the Arab world, to Native American tribes, cultures from around the world and throughout recorded history have valued pearls longer than any other gem.

Pearls are the only gemstones grown inside of a living organism. Pearls are formed within oysters or mollusks when a foreign substance (most often a parasite - not a grain of sand) invades the shell of the mollusk and enters the soft mantle tissue. In response to the irritation, the mantle's epithelial cells form a sac (known as a pearl sac) which secretes a crystalline substance called nacre, the same substance which makes up the interior of a mollusk's shell, which builds up in layers around the irritant, forming a pearl.

There are approximately 8,000 different species of bi-valve mollusks, of which only about 20 are capable of consistently producing pearls. Natural pearls are extremely rare. Because the layers of nacre tend to maintain the irregular shape of the original irritant, natural pearls which are round or spherical in shape are even more rare. Most natural pearls are irregularly shaped.               

In a completely natural state, only a very small percentage of mollusks will ever produce a pearl and only a few of them will develop a desirable size, shape, and color; only a small fraction of those will be harvested by humans. It is commonly assumed that one in ten thousand mollusks naturally produce gem quality pearls. Obviously, if we relied only on nature, ownership of pearls would still be relegated to the wealthiest and pearl producing mollusks would be on the brink of extinction due to over-harvest. As pearls have been prized for thousands of years, this need has led to the development of cultured pearls.

 

Cultured Pearls Defined

A cultured pearl is any pearl grown with the influence of human intervention.

In the early part of the 20th century, Japanese researchers discovered a method of producing pearls artificially. Essentially, the method involves inserting a foreign substance, or nucleus, into the tissue of the oyster or mollusk, then returning it to the sea, allowing a cultured pearl to develop naturally. This practice was already quite widespread culturing hemispherical pearls known as mabe pearls. Kokichi Mikimoto is credited with perfecting the technique for artificially stimulating the development of round pearls in akoya mollusks, receiving a patent for this technique in 1916. Although patented in 1916 this technique has since been improved upon and used extensively throughout the pearling world - no longer simply used to cultured akoya pearls, but freshwater, South Sea and Tahitian pearls as well.

Mikimoto opened the door to a greatly expanded pearl industry in which pearls could be farmed like an agricultural crop. These cultured pearls could now be produced in sufficient quantities to make them available to virtually anyone.

The cultured pearl industry has now far surpassed that of the natural pearl industry. Although a market still exists for pearls gifted to us by nature, these pearls are becoming more and more difficult to find, with rare full strands being auctioned for hundreds of thousands of dollars. Today, purchasing pearls from nearly any store in the world means purchasing a strand of cultured pearls.

Cultured Pearl Formation

A pearl is formed when a small irritant or parasite penetrates and lodges in the mantle tissue of a mollusk. In response, a substance called nacre is secreted, and the creation of a pearl begins. Nacre is a combination of crystalline and organic substances. The nacre builds up in layers, as it surrounds the irritant to protect the mollusk, and after a few years, this build up of nacre forms a pearl.

Natural Pearls vs. Cultured Pearls

Natural pearls, are pearls formed by chance. Cultured pearls have been given a helping hand by man. Today, nearly all pearls are cultured. By inserting a foreign object into a mollusk, pearl farmers can induce the creation of a pearl. From there, the same process of natural pearl creation takes place. The difference is that in this case, the inducement is intentional.

Cultured pearls can be distinguished from natural pearls through the use of x-rays, which reveal the inner part of the pearl.                

The Early Days of Cultured Pearls

Modern-day cultured pearls are the result of discoveries made in the late 19th and early 20th centuries, by Japanese researchers Tatsuhei Mise, Tokishi Nishikawa, and the son of a noodle maker, Kokichi Mikimoto. Although some cultures had long been able to artificially stimulate freshwater mollusks into producing a type of pearl, the pearls produced in this way were generally hemispherical mabes, rather than completely round pearls.

Finally, Producing Round Cultured Pearls

While early on in Mikimoto’s career he focused on mabe production, he eventually produced rounds with a technique involving tissue and bead insertion into the gonad of an akoya mollusk. He patented this technique in 1916.

The Cultured Pearl Industry Is Born

This revolutionized the pearl industry because it allowed the reliable, consistent cultivation of large numbers of quality pearls. The cultured pearl industry of today was born. While natural pearls have widely varying shapes, sizes, and qualities, and are difficult to find, cultured pearls could now be "designed" round from the start. High quality, round pearls could now be produced by the millions; making them available and affordable to everyone.

Today’s Cultured Pearl Industry

The development of pearl culturing took much of the chance, risk, and guesswork out of the pearl industry. It has allowed it to become stable and predictable, fostering rapid growth over the past 100 years. Today, the cultured pearl industry has essentially replaced the natural pearl industry with production of cultured freshwater, South Sea, Tahitian, and of course Mikimoto’s original akoya pearls.

 

Saltwater Pearls Defined

A saltwater pearl is a pearl produced by a saltwater mollusk in a saline environment.

Traditional Saltwater Pearls

Traditionally, most pearls were gathered from saltwater-dwelling mollusks in the Persian Gulf, the Red Sea and the coastal waters of India and Japan. These saltwater pearls were referred to as marine pearls. Natural saltwater pearls are still found, but the yield is too small to account for any significant market share.

Saltwater Pearls Today

Today, any pearls cultured in mollusks inhabiting saline waters are considered saltwater pearls. The three most common types of saltwater pearls are akoya pearls, Tahitian pearls, and South Sea pearls.

Shape of Saltwater Cultured Pearls

Saltwater cultured pearls tend to be more round than freshwater cultured pearls. This is due to the fact that saltwater mollusks are universally bead nucleated. Unless the saltwater pearl is a keshi pearl, it will have a bead core.

Saltwater Perliculture

Saltwater pearls are cultured by prying open the mollusk 2-3 centimeters. A technician then uses a special instrument to make a minute incision on the gonad (reproductive organ) of the animal. A small bead nucleus is inserted into this hole, and a tiny piece of mantle tissue is then placed behind it. The epithelial cells in this mantle tissue grow around the nucleus producing a pearl sac. This is where the pearl grows. This process is the same for all saltwater pearls cultured today.

 

Freshwater Pearls Defined

Freshwater pearls are pearls which grow in non-saline environment in freshwater mussels.

Where Do Freshwater Pearls Come From?

Although the traditional source of pearls has been saltwater mollusks, freshwater mussels, which live in ponds, lakes and rivers, can also produce pearls. China has harvested freshwater pearls in the form of mabe since the 13th century, and has now become the world's undisputed leader in freshwater pearl production. The first record mentioning pearls in China was from 2206 BC. The United States was also a major source of natural freshwater pearls, from the discovery of the New World, through the 19th century, until over-harvesting and increasing pollution significantly reduced the number of available pearl-forming mussels in the US.

The Appeal of Freshwater Pearls

Generally speaking, freshwater pearls are not as round as saltwater pearls, and they do not have the same sharp luster and shine as akoya pearls. However, they appear in a wide variety of shapes and natural colors, and they tend to be less expensive than saltwater pearls, making them very popular with younger people and designers. Also, because freshwater pearls are solid nacre, they are also quite durable, resisting chipping, wear, and degeneration.  

China Leads The World In Freshwater Pearl Production

With a total production of 1,500 tons in 2006, China holds a monopoly over the pearl industry today. Although the birth of the Chinese freshwater pearl industry is traced back to the area around Shanghai, freshwater pearls are now produced in all the surrounding provinces including: Zhejiang, Anhui, Jiangsu, Hubei, Hunan, and Jianxi. Local pearl trade is conducted mainly in the cities of Zhuji (Shanxiahu), Suzhou, Wuxi, Wenling, and Weitang. The largest marketplace for these freshwater pearls is the world's pearl trading hub, Hong Kong.

What Makes Freshwater Pearls Different?

Freshwater pearls differ from other cultured pearls, in that the great majority of them are not bead-nucleated. Freshwater mollusks are nucleated by creating a small incision in the fleshy mantle tissue of a 6 to 12 month old mussel, then inserting a 3mm square piece of mantle tissue from a donor mussel. Upon insertion, the donor, (graft) tissue is twisted slightly, rounding out the edges. What happens after this point is really just speculation. Some believe that this tissue acts as a catalyst in producing a pearl sac thus making the 'nucleation' actual 'activation'. Others believe the tissue molds with the host to create a pearl sac, while still others maintain the tissue is the actual nucleus. Although it is said that a freshwater mollusk can withstand up to 25 insertions per valve, it is common industry practice to perform only 12-16 insertions in either valve, for a total production of 24-32 pearls. The mollusks are then returned to their freshwater environment where they are tended for 2-6 years. The resulting pearls are of solid nacre, but without a bead nucleus to guide the growth process the pearls are rarely perfectly round.

What Makes Today’s Freshwater Pearls So Much Better?

The major increase in quality can be attributed to several factors. The primary jump in quality was accomplished when the industry shifted from the Cockscomb pearl mussel, (Cristaria plicata) to the Triangle shell, (Hyriopsis cumingii) in the middle 1990's. The Cockscomb was responsible for the low-quality rice-crispy pearls of the 1970's and 1980's. Another shift in quality can be attributed to the lower number of grafts inserted into either valve. This number has dropped by an average of 5 per side in the last decade. The turn of the century brought another wave of quality and exotic pearl colors in the form of mussel hybridization.

Japans Freshwater Pearl Industry, A Rough History

The Japanese have a distinguished history of culturing freshwater pearls as well. Lake Biwa was once world renowned for producing high-quality freshwater pearls produced by the Hyriopsis schlegelii, (Biwa pearly mussel) mussel. However, in the mid 1970's pearl farming all but came to a halt due to pollution in this lake that was once synonymous with freshwater pearls. The Japanese tried once again to farm freshwater pearls in Lake Kasumigaura in the last decade, utilizing a bead-nucleated hybrid mussel (Hyriopsis Schlegelii/Hyriopsis cumingii). The resulting pearls have been quite large and unique. The Kasumiga pearl industry had a very short life span, however, with production ceasing in 2006. The industry is once again a pollution fatality of Japanese industry. The remaining Kasumiga pearls are exclusively sold by the Belpearl pearl company.

 

Akoya Pearls Defined

Akoya pearls are bead-nucleated cultured pearls produced in the Pinctada fucata martensii and Pinctada fucata chemnitzii primarily in Japan, China, Vietnam, South Korea and Australia.

What Are Akoya Pearls?

Akoya pearls are cultured in the Pinctada fucata martensii, also known as the akoya oyster. This mollusk is found and farmed primarily in Japan and China. Renowned for their luster, akoya are considered the classic pearl. They are generally white or cream colored, with overtone colors of rose, silver, or cream.

Akoya Pearls, The Perfect Pearl For Jewelry

The akoya oyster is the smallest pearl-producing oyster used in pearl culture today, so akoya pearls also tend to be small, ranging in size from about 2 to 11 millimeters. They also tend to be the most consistently round and near-round pearls, making them ideal in terms of matching for multi-pearl jewelry such as strands and bracelets.   

China Overtakes Japan

In recent years the Chinese have overtaken the Japanese in akoya pearl production. The Chinese began culturing akoya pearls in the 1960's, but had limited success until the late 1980's. While once considered inferior to their Japanese counterparts, China is now producing akoya pearls of qualities that rival that of the Japanese in every quality factor.

Japanese Industry Reaction

Due to the increased pressure of the Chinese competition, many Japanese pearl farmers have focused much of their attention on culturing large akoya pearls, as quality akoya pearls larger than 8 mm are a rare find in China. In lieu of farming smaller pearls, many Japanese factories now import their smaller akoya requirements from neighboring China. The pearls are treated and strung in Japan so that they may still carry the mark 'Product of Japan'. It has been reported that more than 80% of the pearls 7 mm and smaller have come from Chinese farms regardless of whether or not they are sold by Japanese suppliers as Japanese pearls.

 

Tahitian Pearls Defined

Tahitian pearls are bead-nucleated pearls grown in the gonad of the Pinctada margaritifera mollusk in French Polynesia.

Tahitian Pearls – Among The Most Beautiful In The World

Tahitian pearls are produced in the black-lipped oyster ‘Pinctada margaritifera’, in and around Tahiti and the French Polynesian islands. This oyster itself is quite large - sometimes over 12 inches across and weighing as much as 10 pounds - which often results in much larger-than-average pearls. The pearls are unique because of their natural dark colors. Most "black" Tahitian pearls are not actually black, but are instead silver, charcoal, or a multitude of colors with the dominant color being green. Truly black pearls are among the most beautiful pearls in the world, and are extremely rare.

Almost Hunted To Extinction

Not only are the pearls beautiful, but the black-lipped oyster's mother-of-pearl inner shell is also extremely attractive. By the early part of the 20th century, before conservation and repopulation efforts began, the Tahitian pearl oyster had almost been hunted to extinction for its shell alone.

Tahitian Pearls – Not From Tahiti

Although Tahitian pearls are thought by many to be solely a product of Tahiti, this is in fact not true. Tahiti is the commercial center and trading hub for the bulk of the industry, however Tahiti does not have any pearl farms actually located on the island. The farms are instead scattered throughout French Polynesia, as far east as the Gambier Islands, and beyond French Polynesia to the west into the Micronesian Islands. Australia, the Seychelles and Vietnam have all produced black pearls as well, but those cannot be referred to as Tahitian pearls.

Tahitian Pearl Farming Begins

Tahitian pearl farming has much later commercial origins than its other cultured pearl cousins. In the early 1960's a man by the name of Jean-Marie Domard began experimenting with the ‘Pinctada margaritifera’ using Japanese culturing techniques. In 1962, Mr. Domard successfully nucleated 5,000 oysters, and after 3 years harvested more than 1000 high-quality Tahitian pearls.

 

South Sea Pearls Defined

A South Sea pearl is pearl produced by the Pinctada maxima mollusk. They are currently cultured in areas throughout the Indian and Pacific Oceans, primarily in Australia, the Philippines, Indonesia and Myanmar.

South Sea Pearls – Among The Largest In The World

South Sea pearls are among the largest commercially harvested cultured pearls in the world. The average size of a South Sea pearl is 13 mm, with most harvests producing a range of sizes from 9 mm to 20 mm. The South Seas lie between the northern coast of Australia and the southern coast of China. These waters are the native habitat of a large oyster known as Pinctada maxima. This oyster grows up to 12 inches in diameter, and can be nucleated with a much larger bead than other saltwater oysters such as the akoya.

South Sea Pearls Come From Two Varieties Of Pearl-Producing Mollusks

There are two varieties of Pinctada maxima, the silver-lipped and the gold-lipped. The two are distinguished by their distinct coloration of the outer edge of the interior. This type of shell is also known as mother-of-pearl, and is responsible for the coloration of the cultured pearls produced, therefore the name. Unlike the akoya oyster, the South Sea oyster will only accept one nucleation at a time. The oyster is nucleated when it is only about half developed, from 4.7 inches to 6.7 inches in size, or about 24 months old. Although the South Sea oyster will only handle one nucleus at a time, this oyster (like the Tahitian pearl producing Pinctada margaritifera) can be nucleated up to three times over the course of many years.

Why South Sea Pearls Grow So Large

There are four reasons South Sea pearls can grow to such large sizes, dwarfing many of their other saltwater pearl counterparts. These reasons are: the large size of the Pinctada maxima, the size of the implanted bead, the length of time the pearl is left to grow in the oyster, and the oyster’s environment. Due to the size of the oyster, it is able to accept a large bead. The gonad of the Pinctada maxima is several times larger than that of the akoya. Because of this larger gonad, the South Sea oyster deposits nacre around the nucleus at a much quicker rate, especially in warm water, which speeds the oyster’s metabolism.    

The South Seas are also extremely clean, and filled with plankton – the Pinctada maxima's favorite food source. The clean waters and abundant food supply also speeds the nacre production. The growth period for South Sea pearls is also substantially longer than that of the akoya. Akoya pearls are harvested after only 9-16 months, where as South Sea pearls are harvested after a minimum of two years allowing for a larger size.

What Makes South Sea Pearls So Unique

South Sea pearls have several distinct characteristics that are unique to this gem. The nacre is unusually thick, ranging from 2 - 6 mm, compared to the 0.35 - 0.7 mm of an akoya pearl. South Sea pearls have a unique, satiny luster that comes from the rapidly deposited nacre and warm waters of the South Seas. South Sea pearls also have a subtle array of colors; typically white, silver, and golden, that are rare in other pearl types.

 

Cortez Pearls Defined

Cortez pearls are pearls grown in the Pinctada maxima and Pteria sterna mollusks in the Gulf of California.

The History And Comeback Of Cortez Pearls

Emerging once more from the clear waters of the Gulf of California, after decades of protection, is Mexico’s most emblematic gem: ‘New World Black Pearls’. In 1533, the Spanish Conqueror Hernán Cortez sent the very first expeditions into the area to find the mythical “Sea of Pearls”. Soon, the area known as the “Vermillion Sea of Cortez” – officially known as the Gulf of California - produced some of the finest known pearls in the world: the Sea of Cortez Pearl. These prized gems became New Spain’s most important export product, with a price so high that their value was over double of that of all other combined exports to the Old Continent: gold, silver and spices. It was at this moment in time when Mexican black pearls came to be known as the “Queen of Gems and Gem of Queens”, helping to adorn European nobility like never before.

Cortez Pearls Come From Two Species

Sea of Cortez Pearls originate from two species of mollusks that inhabit the Pacific coastline: the “Panamic Black-Lipped Oyster ” or “Madreperla”, (Pinctada mazatlanica) and the “Rainbow-Lipped Oyster ” or “Concha Nácar”, (Pteria sterna) both capable of producing pearls of incredible beauty. The “Rainbow-Lipped Oyster” produces pearls of highly unusual colorations and intense iridescence, thus producing a pearl that is clearly distinguishable from all others.

Cortez Pearls Were Almost Hunted To Extinction

Because of their demand, the natural pearl beds were fished constantly for pearls and pearl-shell, so in 1939 a permanent fishing ban was imposed in order to save the few remaining pearl oyster populations, ending up a 400-year reign for the Gulf of California black pearl. Nonetheless, this period left a clear mark in the history, culture and traditions of northwestern Mexico: a living history of legends, stories of sadness and glory.  

Cultured Production Brings The Comeback Of Cortez Pearls

Nowadays, these cultured pearls are produced in Mexico’s Gulf of California: Bacochibampo Bay, Guaymas, Sonora. Pearl culture began in the region in 1993 as a university research program, but by 1996 a commercial production of 20,000 mabe was achieved. Loose cultured pearl production began in the year 2000. The only marine cultured pearl farm in the Western hemisphere is a medium-sized farm, with some 200 thousand oysters grown in a suspended culture system, with a yearly output of 3-4 kg of cultured pearls, and 5 thousand mabe pearls.

The Culturing Process For Cortez Pearls

The culture process is continuous and has a minimum production term of 4 years: 2 years for the mollusk's grow-out period, and an additional 2 years for the production of cultured pearls. The pearl seeding operation is proprietary, and known only to the original university research team. Each adult, (2 years old, 8 to 10 cm in diameter) Rainbow-Lipped Oyster (Pteria sterna) is seeded with only one American freshwater shell-nucleus, ranging in size from 2.1 to 3.3 bu (6.5 to 10.0 mm) and a piece of mantle tissue from a donor oyster.

Sea of Cortez Cultured Pearl Attributes:

Nacre thickness: is of a minimum of 0.8 mm to a 2.3 mm after the 24 month culture period.

Size: The pearls range in size from 8.0 to 14.0 mm in diameter. Pearls in sizes above 10 mm represent only 5% of a harvest.

Pearl Shape: Baroques make up for the largest yield (71%) while rounds and near-rounds account for only 3%. Semi-Baroque shapes represent 25% of the harvest.

Pearl coloration: Cortez pearls either display an opalescent rainbow-like orient or intense overtones. Body colors range from both warm and cool tones of white, silver-gray, bronze to black, with several overtones of green, purple, blue, gold and violet.

Treatments: The pearls are untreated. After harvest, pearls are soaked in water and pat dried. The Sea of Cortez Pearl (or “Cortez Pearl”) is the only pearl in the gem industry that completely qualifies under the "Fair Trade Gems" protocols.

 

Keshi Pearls Defined

A keshi pearl is a non-beaded pearl formed by accident as a by-product of a pearl culturing operation.

How Keshi Pearls Are Formed

Keshi pearls are formed when the oyster rejects and spits out the implanted nucleus before the culturing process is complete, or the implanted mantle tissue fractures and forms separate pearl sacs without nuclei. These pearl sacs eventually produce pearls without a nucleus.

Keshi Pearls Can Be Salt Or Freshwater

Keshi may form in either saltwater or freshwater pearls. They are generally small in size and, because there was no nucleus to guide the ultimate shaping of the pearl, their shapes vary widely. Keshi come in a wide variety of colors, and tend to have high luster and even rare orient. This is due to their solid-nacre composition.

Keshi Pearls Are Known For Their Luster

Because the oyster has expelled the implanted nucleus of the pearl, the resulting keshi pearl is 100% nacre. This gives it an especially lustrous and shimmering surface quality. Most keshi, in fact, have a greater luster than even the highest quality cultured pearls.               

Keshi Pearls Are Not Considered Natural Pearls

The fact that keshi pearls are solid nacre does not, however, give them the classification of natural pearls. This is because keshi are a bi-product of the culturing process, and not a natural occurrence.

Keshi Pearls Are Now A Very Rare Find

Keshi pearls, especially Tahitian and South Sea keshis, were once quite the bargain, yet beautiful and unique pieces. Today, Keshi pearls are much more rare. This is because Tahitian and South Sea pearl farms are now x-raying oysters to determine whether or not the nucleus has been expelled. When a nucleus-free oyster is found they are then re-nucleated before a keshi has time to form. This practice has made keshi pearls much more of a rare find than they once used to be. The word keshi means "poppy seed" in Japanese, and these pearls are often also referred to as "poppy seed pearls."

 

Mabe Pearls Defined

A mabe pearl is a worked and assembled blister pearl (attached to the shell).

What Is A Mabe Pearl?

A mabe pearl is a hemispherical shaped pearl which is grown against the inside of the oyster's shell, rather than within its tissue. Mabes occasionally appear in nature.

Mabe Pearls Develop On The Shell

Cultured mabes are grown intentionally, by using a hemispheric nucleus, rather than a round one; and by implanting it against the oyster's shell, rather than within its tissue. The pearl then develops in a hemispheric form, with a flat back. While in the oyster a mabe pearl is actually considered a blister pearl not a mabe pearl.

Creating Mabe Pearls

After the blister pearl has developed, it is 'worked' to become a mabe pearl. Blister pearls are ‘worked’ by cutting the pearl out of the shell with a circle-bit drill. The nucleus is then removed and replaced with a resin. The back of the pearl is then capped with a piece of mother-of-pearl to complete the mabe pearl.

Mabe Pearls, Perfect For Jewelry

Cultured mabes are used for such things as rings and earrings, rather than for stringing on necklaces. They tend to be very beautiful with high luster and orient, but are priced much lower than round pearls.

 

Natural Pearls Defined

Natural pearls are calcium carbonate secretions which form within mollusks without human intervention.

Natural Pearls - Accidents of Nature

Natural pearls are formed randomly and really are simple accidents of nature. When a certain type of irritant, such as a parasite, becomes lodged in the tissue of a mollusk, the animal responds by secreting a calcium carbonate substance called nacre to coat the intruder and protect the mollusk. Over a period of several years, this build-up of nacre forms a natural pearl.

Discovery of Natural Pearls is Rare

Natural pearls of any commercial value or desirability, are extremely rare. Instead, since the early part of the 20th century, cultured pearls have supplanted natural pearls as the most common and available pearls.

Cultured Pearls vs. Natural Pearls

Cultured pearls are still real pearls, grown organically inside of oysters in the same way as natural pearls. The difference is, that in the case of cultured pearls, the pearl farmer intentionally stimulates the development of the pearl by inserting a "nucleus" into the oyster. Thus, the formation and discovery of the pearl are no longer left to chance.  

Natural Pearls Today

Although natural pearls are found primarily in older jewelry from estate sales and auctions, their popularity is making a comeback. Colorful abalone and conch pearls have begun to find favor in exotic, unique designs. Recently, in 2007, a double strand of 68 perfect natural pearls known as the ‘Baroda Pearls’ was auctioned at Christie’s Auction House for $7 million.

 

Pearl-Producing Mollusks (Pearl-Producing Oysters)

Mollusks represent one of the world's earliest forms of animal life, and date back over 550 million years. Actual pearl-producing mollusks first appeared 530 million years ago, when mollusks developed shells. The scientific discoveries of pearl producing mollusks illustrate the intrinsic and irrefutable value of the world’s most historically treasured organic gem.

 

Scientific Classification Of Pearl-Producing Mollusks

Mollusks encompass the second largest phylum of the animal kingdom (there are approximately 128,000 different species). The phylum is divided into two subphyla, only one of which contains species that produce pearls. Conchifera is the name given to the subphylum of mollusks that produces pearls. In this discussion, a pearl-producing mollusk is one that will produce jewelry-quality pearls.

Which Are The Pearl-Producing Mollusks?

Although any of the mollusks within the Conchifera subphylum can produce pearl-like formations, most of them are not pearl producing mollusks. It is actually the fourth class of the Mollusca phylum, bivalia, which is the most important for the formation of pearls. Other notable classes that produce pearls are the gastropods (2nd class) and cephalopods (5th class). According to the scientific classification system developed by Carl Von Linne in 1758, each species of mollusk is given a two-part Latin name: the genus and the species. Sometimes a third part will describe sub-species, which may represent regional differences among a species of pearl-producing mollusk. Although the shell of each mollusk is important for identification and critical to pearl formation, it is actually the inner soft body (mantle) of the mollusk that scientifically defines the species.

The Evolution Of Pearl-Producing Mollusks

The figure below illustrates the evolutionary tree for the Phylum Mollusca. Not all pearl producing mollusks belong to a single family or group. Pearl producers are distributed across the evolutionary tree. The boxes that are shaded indicate classes of Conchifera that exhibit the pearl producing compound of nacre. Nacre is the tissue that lines the shell and creates the unique luster of pearls.

The widespread presence of nacre indicates one of two points:

•Nacre is a primitive characteristic that has been lost and regained throughout the mollusks’ evolutionary history.

•The composition of nacre is unique within certain branches, and certain forms of nacre facilitate pearl production.

If the latter were true, it could explain why certain species of mollusk produce more pearls than others.

Pearl-Producing Mollusks Are Not Oysters

Most pearl-producing mollusks are bivalves, meaning their shells have two halves connected by a hinge, like a clam. Although there are approximately 20,000 species of bivalve mollusks, only relatively few of these species are used in pearl culturing to create commercial pearls. The term “pearl oyster”, commonly used in the pearl trade, is in fact incorrect - pearl producing mollusks are not oysters. Though edible oysters can occasionally produce a pearl, they are of no commercial significance. Furthermore, for the safety of our teeth, pearls in edible oysters are undesirable. Commercial farming beds for oysters are in fact shut down if pearls persist to form within them.

The Anatomy Of Pearl-Producing Mollusks

The anatomy of a bivalve mollusk facilitates the production of pearls. Unlike a closed snail, or gastropod shell, a bivalve clamshell is open and water filled, leaving more room for pearls to form. Most bivalves are also passive filter feeders - meaning they maintain an open relationship with the environment by constantly circulating water in order to supply food. This process is critical for pearl production, since most natural pearls are formed as a reaction to a parasite or foreign object within the shell. The open relationship of the bivalve structure increases the probability of foreign objects and creatures to enter, and the possibility of a natural pearl to develop.

Gastropods Are A Different Pearl-Producing Mollusk

In contrast, gastropods do not rely on water flow for food and are usually predators who deliberately scrape food from rocks or from prey. Gastropods are also mobile and able to expel foreign particles before they become a pearl's nucleus through movement. Mobility has been a difficult factor in culturing abalone pearls. Abalone pearls come from large, ear-shaped snails, whose movement often expels inserted nuclei. Abalone pearls have beautiful blue-green nacre and display gorgeous rainbow iridescence. Because of their irregular shapes and liquid iridescence, abalone pearls epitomize the beautiful canvas of nacre that creates singular beauty in each pearl.

 

Pearl Farming Defined

Pearl farming is the industry responsible for grafting pearl mollusks and producing cultured pearls. These cultured pearls make up nearly 100% of the pearls sold today. Natural pearls now only account for less than 1/1000th of a percent of the pearls on the market today.

What Is Pearl Farming?

Cultured pearls are grown on what are known as pearl farms. Several thousand oysters are nucleated and then cared for during the 2-5 years required for a pearl to grow and develop. Like any other form of farming, pearl farming can be as dependent on luck as it is on skill. An entire bed of oysters can be completely devastated by unpredictable and uncontrollable factors, such as water pollution, severe storms, excessive heat or cold, disease and many other natural and man-made phenomena. Although pearl farmers attempt to control as many of these variables as possible, pearl farming can indeed be a risky business!

Modern Pearl Farming Techniques

The first step in the pearl production process is to obtain oysters to be nucleated. In the early days of the cultured pearl industry, oysters were simply collected from the sea. Although some farmers continue using this method today, many use the more modern practice of breeding their own oysters. To do this, the pearl farmer collects oyster sperm and eggs from high-quality oysters already on the farm. The sperm are used to fertilize the eggs, and so create a new generation of oyster larvae.

How Oysters Are Raised In Pearl Farming

The larvae are allowed to float freely in the water, under controlled conditions, until they are a few weeks old. In the wild, the larvae would then attach themselves to a rock or similar object, so the farmers provide “collectors” for this purpose. Over a period of a few months, the larvae develop into baby oysters. They are generally then moved into a separate "nursery" area of the farm. Here they are tended for around 1-2 years, until they have grown sufficiently large to be nucleated.         

The Process Of Nucleation In Pearl Farming

The process of nucleation is a surgical procedure, whereby a foreign object is implanted into the oyster. This object causes irritation, which the oyster counteracts by secreting nacre to surround the object; this produces the pearl.

How Oysters Are Raised In Pearl Farming

The larvae are allowed to float freely in the water, under controlled conditions, until they are a few weeks old. In the wild, the larvae would then attach themselves to a rock or similar object, so the farmers provide “collectors” for this purpose. Over a period of a few months, the larvae develop into baby oysters. They are generally then moved into a separate "nursery" area of the farm. Here they are tended for around 1-2 years, until they have grown sufficiently large to be nucleated.

Saltwater Nucleation In Pearl Farming

Two basic methods of nucleation are used. Saltwater oysters are generally nucleated using a "bead", prepared from mother-of-pearl. First, the bead is surrounded by a small piece of mantle tissue taken from a donor oyster. The bead and tissue are then implanted into the oyster's gonad. The bead serves as a mold, or nucleus, around which the pearl develops. The resulting pearl will contain the bead at its center and will tend to develop in the same general shape as the original bead. The bead can be detected in the final pearl by x-rays.

Freshwater Mussel Grafting In Pearl Farming

Freshwater mussels are generally grafted using a piece of mantle tissue only, without a bead. This small piece of mantle tissue is placed into an incision in the host mussel's mantle instead of the gonad. Both sides of the valve can accept grafts, and an average freshwater mussel will produce 24 to 32 pearls per culturing cycle.

The Pearl Is Now Allowed To Grow

After nucleating, the oysters are given a few weeks to recover from the surgery. During this time, some of the oysters may reject and expel the implanted nuclei; others may become sick or even die. Most, however, will fully recover. The oysters are then placed in cages or nets and moved into the oyster bed, where they will be tended as the pearls develop. Depending on the type of oyster, this process can require anywhere from a few additional months to several more years!

Finally, The Pearls Are Harvested

After the pearls have been allowed to develop fully, they must be harvested. After the pearls are extracted from the oysters, they are washed, dried, and sorted into general categories. Sometimes, the pearls are polished by tumbling in salt and water. The pearls are then sold to jewelers, manufacturers, and pearl dealers

Pearl Nucleus Defined

A pearl nucleus is the bead implanted into the the gonad of a marine mollusk or into an existing pearl sac in the mantle of a freshwater mussel around which nacre deposition occurs.

The Pearl Nucleus and Nucleation

Pearl farms vary greatly depending on the type of pearls being produced. But the one thing all pearl farms share is the process of creating the pearl nucleus, and nucleating the pearl. Every pearl produced commercially today, except Keishi pearls, are a result of nucleation, whether it be beaded or tissue nucleation.

The Units of Pearl Nuclei

The discovery of the attributes of the freshwater mussel shell in nucleus production is attributed to Mikimoto, who experimented with many materials. Due to this Japanese influence, nuclei are still sold today universally utilizing the ancient Japanese unit of length, the 'bu'. One bu is equivalent to 1/100th of a shaku. The metric value of 1 bu is 3.03mm, and the English value is .1193 inch.

Saltwater Pearl Nucleus

The nucleus used in all saltwater pearl farming today is a mother-of-pearl bead. This bead is composed of mussel shell that has been cut, rounded, and polished. A nucleus of high quality will be white, without calcium carbonate streaking (striation or banding), that may show through the pearl nacre.     

How the Nucleus Creates a Pearl

The first step in nucleating saltwater mollusks is making a small incision to the gonad. The mother of pearl nucleus is inserted into this incision, which is then followed with a very small piece of mantle tissue from a donor mollusk. The mantle tissue is placed between the mother of pearl bead and the gonad, with the side containing epithelial cells facing the nucleus. These epithelial cells are the catalyst of the pearl sac. The pearl sac then grows around the nucleus and begins to deposit nacre. This nacre layering is what creates the beauty of the pearl.

How Many Times Can a Mollusk Be Nucleated?

Saltwater mollusks will only produce 1-2 pearls per typical nucleation. Akoya can be nucleated with up to 5 beads, but the use of only 2 is most common. The akoya dies at harvest. South Sea and Tahitian mollusks (Pictada margaritifera and Pinctada maxima) accept only one nucleus at a time, but as they do not die at harvest, they may be nucleated several times. If a particular mollusk has been successfully nucleated several times and consistently produces fine pearls, the mollusk is often returned to the wild to strengthen the genes of future generations of spat.

Freshwater Pearl Nucleation

Freshwater pearls must also be nucleated, but in a different fashion. In lieu of the mother-of-pearl bead, freshwater pearl farmers nucleate their mussels with small pieces of mantle tissue. These mantle tissue pieces are not placed in the reproductive organ of the mussel, but in the fleshy mantle tissue. Because the mantle tissue is large and located on either side of the shell, each mussel can withstand many insertions. Most mussels receive 12 to 16 insertions on either side of the valve for a total of 24 to

32. The large number of freshwater pearls produced per mussel accounts for some of the diminished value between freshwater pearls and their saltwater cousins. But, because the mantle tissue is dissolved into the pearl-sac, freshwater pearls are solid nacre

 

The Pearl Harvest

The pearl harvest is the pay-off for years of investment and hard work. The harvest most-often happens during the winter months as the host mollusks' metabolism slows and nacre platelets become thinner as deposition decreases. This has proven to increase the luster upon harvest.

Pearl Harvest Times Vary

Cultured pearls are harvested from the pearl farms after a nucleation period of 8 months to 6 years. Akoya pearls are typically cultured from 8 months to 2 years, while freshwater, South Sea and Tahitian pearls are cultured from 2-6 years.

Pearl Harvests Usually Occur In Winter

Pearls are almost universally harvested in the winter. During these colder months, the metabolism of the host oyster has decreased, slowing the nacre deposits around the pearl. While slower nacre deposits are reflected in less growth during these colder months, it also enhances the outer layers of the cultured pearls' nacre.

Pearl Harvest Times Vary

The actual harvest begins when the pearls are brought to shore from their mooring positions. The pearls are then opened individually and the pearl or pearls extracted. In the case of akoya oysters and freshwater mussels, the shell and meat are discarded or used for another purpose. South Sea and Tahitian oysters, however, are harvested in much the same fashion as the nucleation. These oysters are opened very slightly, the pearl is extracted, and another nucleus is placed into the preformed pearl sac. This oyster than goes through another recovery period and is placed back into the sea to grow another pearl.  

After The Pearl Harvest, The Pearls Are Cleaned Before Being Treated

After all the pearls are harvested they are then cleaned of debris and polished, to bring out a higher luster.

Different Treatments After The Pearl Harvest

After the pearls have been cleaned and polished, they are sorted by quality and size categories. Many of these pearls may then go through further treatment, depending on the type of pearl and the factory preparing the finished goods. The pearls may be bleached, heat treated and pinked (soaked in red dye to enhance the pink coloration of the pearls).

What Determines A Good Pearl Harvest

A good harvest is determined by the number of marketable pearls produced in relation to the number of oysters that were nucleated. The percentage of pearls that are considered high-quality will determine the success of the enterprise. On average, only half of the pearls will be marketable, and less than 10% of these will be considered top-quality.

 

Pearl Treatments Defined

Pearl treatments are defined as any action other than polishing which alters a pearl's appearance which may include; irradiation, bleaching, heating, filling, waxing, drilling, cutting and working.

Pearl Treatments After Harvesting

After harvest, pearls are always processed in one way or another. Akoya and freshwater pearls are routinely bleached, and all pearls are cleaned and polished before sale. However, there are treatments that should be noted with cultured pearls that change the aesthetic qualities of the gem.

Different Kinds Of Pearl Treatments

When a low-quality cultured pearl is cleaned and polished, and still does not have a good luster, the farmer is left with a few options. He can sell the pearl at a steep discount; dispose of the pearl; peel the nacre to sell it, then reuse the nucleus; or apply a treatment to the pearl that will change its appearance. If the pearl is a good candidate for treatment, this is the most common and economically sensible approach for the farmer.

There are three main treatments that low-quality pearls undergo:

•Dyeing: The use of silver nitrate or other organic dyes to darken the nacre of the pearl.

•Irradiation: The use of gamma rays to darken the nucleus of the pearl in akoya pearls and the nacre layers in freshwater.

•Luster treatments: A pearl is heated and then cooled or a coating treatment placed on the surface of the pearl to artificially enhance the lustre. This is also referred to as "maeshori".

Using Silver Nitrate In Pearl Treatments

Silver nitrate has been used for many decades to darken the appearance of pearls. The chemical penetrates the layers of nacre and has a chemical reaction with light and hydrogen sulfide gas to create a rich black color. If the farmer wishes to create colors other than black, he may also use organic or inorganic dyes to produce another color variation. This is a very popular treatment done to freshwater pearls, as the lower values give farmers more opportunity to experiment. Akoya pearls are also routinely "pinked" to enhance a more desirable rose overtone. 

Using Irradiation In Pearl Treatments

Irradiation has differing effects on freshwater and saltwater cultured pearls. The gamma rays do not affect the nacre layers of saltwater cultured pearls, but in fact darken the nucleus of the pearl. An irradiated saltwater pearl appears to be gray or blue. The nacre of freshwater pearls, on the other hand, when affected by gamma rays can become very dark. Some of these freshwater treated pearls will also have an intense metallic sheen and iridescent orient over their surface.

Using Coating In Pearl Treatments

Coating a pearl to enhance its luster is not widely practiced and is greatly frowned upon. This coating is equivalent to a coat of clear nail polish. Although the luster may appear to be fine, the coating may eventually chip or peel, leaving a low-luster pearl visible beneath the surface. This is a treatment to watch for, as dishonest pearl dealers have passed these pearls on to unsuspecting consumers in the past.

How To Detect Pearl Treatments

Although nearly all pearls on the market today have been treated in some way, it can be difficult to detect pearls treated to change color. One method of detecting dyed or irradiated pearls is to check the matching of the strand. A strand of natural color pearls will typically vary slightly from pearl to pearl. A perfectly consistent strand may be evidence of treatment. By peering down the drill hole of a dyed pearl, one may also be able to see concentrations of color. This residue is left from the dying process. When looking down the drill hole of an irradiated pearl, one may be able to detect a darkened nucleus. This is strong evidence of gamma-ray treatment.

How To Detect Luster Pearl Treatments

Luster treatments are much harder to spot. The most basic method, however, is to compare an untreated strand with the suspect strand under at least 50X magnification. The untreated pearl will have a scaly nacre surface. The coated pearl will have a smooth, glass-like surface

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