A pearl is a reaction to an irritant within a mollusk. Pearls are formed when the mollusk secretes thousands of very thin concentric layers of nacre, a secretion of calcium carbonate (aragonite and conchyolin) in a matrix that eventually coats an irritant, either man made or natural. The thin circumferential lamellae of nacre intersect the external surface of the pearl to create a 'thumbprint pattern' that characterises the surface of nacre.
Pearls form inside a mollusk which is an invertebrate with a soft body, often protected by a shell such as a clam, oyster or mussel. Any mollusk is capable of producing a pearl, although only those mollusks that have shells lined with nacre produce pearls that are used in the jewellery industry.
Cultured pearls are real, genuine pearls that are formed inside a living oyster with human intervention. When a nucleus is surgically implanted in the oyster's flesh, the oyster recognises it as an irritant and begins to coat it with smooth layers of nacre. Over time, the growing pearl gets completely covered with the beautiful iridescent substance we call nacre, or mother-of-pearl. All pearls sold today are cultured pearls, with the exception of vintage estate jewellery and heirloom pieces that are more than 80 years old.
Natural pearls, on the other hand, are formed naturally by free-range "wild" oysters living at sea without any encouragement from humans. When a natural irritant such as a fragment of shell, a scale or a parasite becomes lodged inside an oyster or mollusk, it gets coated with layer upon layer of nacre. Contrary to popular belief, grains of sand do not form pearls. If sand were enough of an irritant, our ocean floors would be littered with millions of natural pearls! Natural pearls are actually very rare, mostly because pearl-producing species of mollusks were nearly hunted to extinction with most natural beds of pearl-bearing oysters depleted by over-harvesting in the 18th and 19th centuries. Today, natural pearls are extremely rare. Only 1 in about 10,000 wild oysters will yield a pearl and of those, only a small percentage achieve the size, shape and colour desirable to the jewellery industry.
What is a Pearl Producing Mollusk?
Mollusks represent the earliest forms of animal life and date back 550 million years. Pearl-producing mollusks first appeared 530 million years ago when mollusks developed shells. Mollusks are invertebrates with a soft body often protected by a shell such as clam, oyster and mussel. Pearls are organic gemstones which form inside a living pearl-producing mollusk. Nacre is formed from iridescent layers or columns of flattened crystallised calcium carbonate, in the form of the mineral aragonite (ah-RAG-uh-nite), secretions over the irritant. These microscopic aragonite crystal layers, called platelets, are held together by conchyolin (kon-KY-uh-lin), an organic binding agent. The thin circumferential lamellae of nacre intersect the external surface of the pearl to create a 'thumbprint pattern' that characterises the surface of nacre.
Pearl oysters are members of the phylum Mollusca and belong to the class Bivalvia. Most pearl producing mollusks are bivalves, meaning their shells have two halves connected by a hinge (like a clam), a soft body with a small foot, a byssal gland and paired gills. Most bivalves are also passive filter feeders - meaning they maintain an open relationship with the environment by constantly circulating water through their shell in order to support their food supply. The anatomy of a bivalve mollusk facilitates the production of pearls. The mollusk opens its shell slightly to allow water to enter its body as it extracts microscopic food particles from the water. The open relationship of the bivalve structure increases the probability of the entrance of foreign objects and creatures. This is critical for pearl production since most natural pearls are formed as a reaction to a parasite or foreign object within the shell.
A pearl-producing mollusk can live in freshwater or saltwater. Freshwater mollusks are referred to as mussels while saltwater mollusks are referred to as oysters. While the name "pearl oyster" suggests a close relationship with other types of oysters, pearl oysters are actually a distinct species from edible oysters and have important anatomical and behavioural differences. There are a small number of mollusks capable of producing a pearl and only those mollusks that have shells lined with nacre (NAY-kur), the pearlescent substance inside the animal's shell) produce the pearls used in the jewellery industry.
Pearl oysters feed on small algae found in the water column. The gills in bivalves are large and tiny hair-like cilia on the gills are used to remove small particles from the water. Both adults and larvae feed on algae and other small organisms. Clear tropical waters contain limited amounts of algae. Therefore, a large amount of water must be filtered daily in order for the pearl oyster to obtain sufficient food. This is the reason importance is placed on not crowding pearl oysters on the farm and for keeping the shells clean from organisms that compete for food.
Pearl oysters are protandric hemaphrodites which means that most are first male, then female. The male phase usually occurs during the first 2 to 3 years of life, with the change to the female phase in later years. Pearl oysters have been reported to live as long as 25 years. Pearl oysters reproduce by releasing millions of eggs or sperm into the water column where fertilisation occurs randomly. In less than 24 hours, the fertilised egg develops into a trocophore larva, a free-swimming organism. The larvae remain suspended in the water column for 2 to 3 weeks before undergoing metamorphosis, changing into an attached juvenile "spat". Shortly before metamorphosis, the larva develops an enlarged foot and an eye-spot. The foot remains after metamorphosis and the young spat retains the ability to move about for several months even after it attaches itself to a hard substrate. Pearl oysters can attach and reattach themselves using the byssus.
Sometimes a natural pearl forms when an irritant, such as a fragment of shell becomes lodged inside the mollusk when it is feeding, or a parasite drills through the shell. To protect itself, the mollusk forms a sac around any irritant or invader that managers to get caught up inside its body. This sac secretes nacre to cover the irritant and, over time, the growing pearls are completely covered with the beautiful iridescent substance we call nacre, or mother-of-pearl. The nacre and sac materials are made by the mollusk's mantle, the layer of tissue cells that surround the body of the mollusk and lines the shell. The mantle tissue cells that make up the pearl sac are called epithelial (ep-uh-THEE-lee-yuhl) cells.
One commonality all cultured pearls share is the nucleus. Every pearl produced commercially today except naturally forming keshi pearls and pearls from Bahrain will have been nucleated. The nucleus used in all pearls farmed in saltwater today is a mother-of-pearl bead made from freshwater mussel shells found in North America. This bead is made from an oyster shell that has been cut, rounded, and polished. A nucleus is surgically implanted in the oyster's gonads or mantle lobe together with a small section of mantle tissue. Implanting a bead alone will not stimulate pearl formation. The epithelial cells - mantle tissue- play a vital role in the pearl formation process. As the oyster recognises the nucleus as an irritant, it forms a sac around the irritant before coating it with smooth layers of nacre. Pearl farms now produce all the cultured pearls used in the jewellery industry today, and, while they are real, genuine pearls formed inside a living oyster, they are produced with a little human intervention.
Saltwater oysters are nucleated by opening the shell a mere 2 to 3 centimetres and making a minute incision in the gonad - the oyster's reproductive organ. 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 oyster. 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 grows around the nucleus and begins to deposit nacre. This nacre layering is the beauty of the pearl.
Saltwater oysters will only produce 1 to 2 pearls per typical nucleation. Akoya oysters can be nucleated with up to 5 beads but the use of only 2 is most common. The Akoya oyster dies at harvest. South Seaoysters (Pinctada 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 oyster has been successfully nucleated several times and consistently produces fine pearls, the oyster is often returned to the wild to strengthen the genes of future generations of spat.
An oyster's pearl sac will secrete nacre on nearly any solid object. This has led to countless attempts to nucleate oysters with material other than oyster shell. Success has been limited, however, and oyster shell is still the main staple of the pearl farmer as it has been since the early 1900s. The reasons nuclei of non-standard composition has been so quickly rejected in the past is because the density of the nucleus must exactly match, or be extremely close to the density of the host mussel. In order for the pearl to expand and contract in different environments, the nucleus must expand and contract in a compatible fashion. This is known as the thermal coefficient of expansion. The nuclei must also resist cracking, hold a high shine and remain stable over long periods of time. The material that best fits these criteria is the shell of the Mississippi freshwater mussel from the Unionidae family. This mussel has the added attribute of a thick shell, especially in the joint where the bivalve connects. This thick shell enables harvesters to create large nuclei to be used in culturing larger pearls.
Pearl Nucleus Composition
The nucleus of a pearl, although it is not typically visible in a harvested pearl, is extremely important in the culturing process. The nucleus is the seed that impregnates the oyster and produces the gem, although the process is not complete unless a small piece of mantle tissue is inserted with the bead.
The bead material used to create the nucleus is almost exclusively derived from freshwater mussel shells found in the rivers of North America. The shell harvested from these rivers is typically first transported to Asia to be worked. This process involves cutting the thick portion of the shells near the hinges into strips then into cubes. These cubes are then shaped into perfect spheres by grinding, tumbling and polishing. These finished nuclei are then separated by size and quality. The finished product falls into different quality ranges in a similar fashion as the actual pearl.