Pearl Science

Pearls are created in the mantle folds of certain breeds of mollusk. In these breeds, the epithelial cells of the mantle, which is soft tissue inside the cell, secretes nacre, also called mother of pearl. Usually, this substance just lines the inside of the shell, but sometimes, foreign matter gets trapped in the folds of the mantle. There, the speck of matter gets covered in subsequent layers of nacre, making it a pearl.

Nacre - What Pearls Are Made Of

The nacreous inside of an oyster shell

Nacre, or mother of pearl, is what Pearls are made of. It is a composite material that is made up of alternating layers of microscopic hexagonal platelets of a mineral called aragonite and thin sheets of elastic biopolymers.

Aragonite is a specific crystal structure of calcium carbonate (CaCO3). Specifically it is in what is knows as an orthorhombic crystal system which are rectangular crystals. It is hard and can tend to be brittle. On the other hand, elastic biopolymers, which are chains of organic substance, such as proteins and carbohydrates, are soft and flexible.

This combination of hard and soft coatings creates a durable material that the mollusk uses line the inside of its shell, make it smooth and sealed, in turn, making it resistant to parasites. Also, the nacre can be used to entomb parasites and foreign debris that find their way in to further protect the mollusk. This is known as encystation and is, of course, the process that makes a pearl.

Nacre is made extra strong due to the layout of the aragonite. The platelets are arranged in an alternating pattern similar to brickwork. As a result, it is more resistant to cracks.

The aragonite platelets are also responsible of nacre's iridescence. The thickness of each platelet varies slightly and each thickness interferes with different wavelengths of light. Consequently, nacre reflects different colors of light when viewed from different angles.

Mollusks - Pearl Makers

Mollusks make pearls. In fact, every mollusk is capable of making pearls, however only pearl oysters and freshwater pearl mussels can make nacreous pearls. All the others, while technically being pearls, are considered valueless as gemstones and are referred to by gemologists as calcareous concentrations.

Oysters

A Black-lip oyster

Only certain species of oysters can produce nacreous pearls. All of them, however, belong to the Pteriidae family, which is comprised of medium to large saltwater clams. This is a different family from the kind of oysters that people can eat, which is the family Ostreidae, or true oysters.

The genus Pinctada, also known as pearl oysters, are the members of the Pteriidae family that can produce gemstone quality pearls. Furthermore, among this Pinctada, only certain species produce commercially viable pearls. Each species produces different sizes and colors of pearls.

Pinctada radiata, or the Gulf Pearl Oyster, is found in Mediterranean, Persian Gulf, and Red Sea. These produce smaller pearls that are usually silver or yellow.
Pinctada margaritifera, or the Black-lip Oyster is found in parts of the Indian Ocean, parts of the South Pacific, near Baja California, and in the Gulf of Mexico. These produce Black South Sea pearls and Tahitian pearls.
Pinctada maxima, or the White-lip Oysters is found in the South Pacific. These produce White and Golden South Sea pearls.
Pinctada fucata, or the Akoya Pearl Oysters is found in the Indian Ocean and the Western Pacific. These produce Akoya pearls.
Pinctada albina, or the Shark Bay Pearl Oyster is found in the waters around Australia. These produce Mabe pearls.

Even among oysters that can produce pearls, naturally formed pearls of value are still rare. Often, hundreds of oysters must be searched to find a single natural pearl. In order to search an oyster for a pearl is must be forced open, killing it in the process. As a result, most pearls are cultured in pearl farms by planting a nucleus in the Oyster.

Mussels

The interior of a freshwater pearl mussel

Various types of freshwater mussels can also produce gemstone quality pearls. They all belong to the order Unionida are in one of two families either the Unionidae or the Margaritiferidae. One particular species, the Margaritifera margaritifera, actually carries the common name Freshwater Pearl Mussel. Because of over-hunting for pearls however, margaritifera is an endangered species. Most freshwater pearls are produced in China but pearl mussels also exist in Japan and North America.

Margaritifera margaritifera, or the Freshwater pearl mussel or European Pearl Mussel can be found in Northern Europe, though it is endangered.
Hyriopsis schlegelii, or the Biwa Pearl Mussel is found in Lake Biwa Japan. It is also nearly extinct.
Cristaria plicata, or the Cockscomb Pearl mussel is found in China and is used to make "rice crispie" pearls.
Hyriopsis cumingii, or the Triangleshell Pearl Mussel is also found in Chine. It produces slightly higher quality pearls than the Cockscomb.
Lampsilis teres, or the Yellow Sanshell Pearl Mussel is found in North America.
Quadrula ndulata, or the Wartyback Pearl Mussel is found in North America.
Megalonaias nervosa, or the Washboard Pearl Mussel is found in North America.
Fusconaia ebena, or the Ebonyshel Pearl Mussel is found in North America.
Pleurobema cordatum, or the Pigtoe Pearl Mussel is found in North America.
Quadrula Metanevra, or the Monkeyface Pearl Mussel is found in North America.