Pearl (Pernulo) is an ancient organic gemstone, mainly produced in the Pteriidae mollusks of the order Anisomyria. It is rich in variety, shapes, and colors, and is commonly used as jewelry, medicine, and food in our daily lives. According to the researches from geology and archaeology, pearls existed on Earth 200 million years ago (http://www.nimrf.net.cn/index.action). China is one of the earliest countries in the world to discover and utilize pearls, and also the earliest country to artificially cultivate pearls. It is truly an ancient country of pearls in the world. As early as over 6000 years ago, there were records in China of collecting and applying pearls. Gezhi Jingyuan · Zhuangtai Ji recorded the historical fact that King Wen of Zhou Dynasty decorated his hair with pearls, and “Huaiyi Binzhu (the Huaihe River Basin is rich in pearls)” was also recorded in Shangshu · Yugong. These all indicate that the history of pearl picking in China began as early as the Xia and Yu dynasties. What’s more, Shi Jing (the Book of Poetry), Shanhai Jing (Classic of Mountains and Rivers), Erya (Literary Expositor), and Zhouyi (the Book of Changes) also contain many records about pearls.

And people are not limited to simply raising, collecting, and applying pearls. They have found that accurately understanding the mechanism of pearl formation can help in subsequent artificial pearl cultivation and provide scientific basis for cultivating higher quality pearls. Since the end of the 18th century, a large number of scientific workers have begun to study the definition and formation mechanism of "pearls" at the true scientific level. More than 100 years ago, scientists cut pearls in half to be directed against the prevailing idealism myths and legends, and found that the nucleus was trematode, the larva, head and egg ball of tapeworm, and even sand grains, so they came to the conclusion that parasites formed pearls. Subsequently, through research some people believed that pearls are based on the remains of parasites as nuclei, forming so-called pearl sacs around them. The pearl sacs secrete nacre and attach to the nucleus, forming pearls. In 1913, Ervidus injected the separated mantle epidermal cells into mantle epidermal cells to form pearl sacs, and achieved the success of artificial cultivation of non-nucleated pearls. In 1947, Masai Matsui and others in Japan found that when the epithelial cells of the mantle had pathological changes, they would have abnormal proliferation, which resulted in wrinkles and depressions, forming pearl sacs and producing pearls. Based on the folding principle of artificially cultivated Buddha beads in the Song Dynasty of China, Japanese scholars used shell spherical nuclei to coat the mantle film and insert them into the mantle to form free round nucleated pearls.

Finally, we define pearl as a spherical or other shaped object formed by a part of cells in the mantle of shellfish for some reason to form pearl sacs in the shellfish body and secrete materials similar to shells, which are deposited around a common core. Based on this, this review provides an overview of the causes, structures, appearance characteristics, and physiological and biochemical characteristics of pearls, providing a scientific basis for improving the quality of cultured pearls in China and the subsequent research and development of new composite materials.

1 The Causes of Pearl Formation

1.1 External causes of pearl formation

The external causes of pearl formation mainly include two formation mechanisms. One is that when the mantle of shellfish and mussel is invaded by foreign bodies (such as sand grains, parasites, etc.), the shell starts its self-protection mechanism. The epidermal cells of the mantle at the stimulated place take the foreign body as the center and sink into the connective tissue of the mantle. Some of the trapped epidermal cells of the mantle constantly split and wrap themselves, forming a cystic structure with the foreign body as the core - pearl sac. The cells of the pearl sac secrete nacre, continuously enveloping foreign objects layer by layer. The nacre thickens over time, ultimately forming a "nucleated pearl" (Figure 1).

Figure 1  Process of nucleated pearl formation

Another formation mechanism is that when the mantle of shellfish and mussel is invaded by foreign bodies, the outer skin cells of the mantle feel external stimulation, and the outer skin cells are trapped in the connective tissue of the mantle after abnormal proliferation, and then the outer skin cells continue to divide to form multiple pearl sacs, secrete nacre, and then form pearls. This type of pearl without foreign matter nuclei is called a "non-nucleated pearl".

1.2 Internal causes of pearl formation

The internal mechanism of pearl formation is related to the pathological changes that occur inside the mussel. The outer skin cells of the mantle of shellfish and mussel are effectively stimulated due to pathological changes. The stimulated part of the epidermis undergoes cell division and then dissociates, immediately wraps the organic substances (keratin) secreted by itself, and gradually sinks into the connective tissue of the mantle at the same time. The outer skin cells continuously divide to form a pearl sac, meanwhile, they secrete nacre to continuously cover and form pearls. This type of pearl formed due to pathological changes is also known as a "non-nucleated pearl" because of the absence of foreign objects as nuclei.

Today's cultured pearls are based on such principles. By cutting small pieces of live epithelial cells (referred to as cell fragments) from the mantle of pearl raising mussels, they are implanted into the mantle connective tissue of the mussel together with the artificial nucleus prepared from the shell of the mussel. The implanted cell fragments rely on the nutrition provided by the connective tissue (artificial nucleated pearls are only implanted with cell fragments when performing operations on the outer mantle), they rapidly proliferate around the artificial nucleus to form pearl sacs and secrete nacre, thus, generating artificial nucleated pearls.

2 The Structural and Appearance Characteristics of Pearls

2.1 Structure of pearl mussels

The shell of pearl mussel is large, thick, and solid, with a long oval shape. The shell can reach a length of 180 mm, a height of 70 mm, and a width of 40 mm. The two shells are swollen, with a dark brown or nearly black surface and glossy spots (Figure 2).

Figure 2 Appearance (A) and interior (B) of freshwater pearl mussel Note: A mussel has a shell composed of one or two hinged parts to protect the soft body inside the animal. The outer surface is usually yellow brown when young, and gradually darkens with age; The old part of the shell often corrodes; The inner surface is pearl white, sometimes with a charming rainbow color

2.2 Appearance characteristics of pearls

The external form of pearls is the shape they present, and people roughly classify their shapes into two kinds: regular and irregular (Figure 3). Circular, semi circular, elliptical, water droplet shaped, pear shaped pearls, etc. belong to regular shaped pearls. In addition to the above-mentioned regular shaped pearls, other shaped pearls are collectively referred to as irregular pearls or baroque pearls.

Figure 3 Pearls of different shapes

Pearls appear transparent to semi transparent. The refractive index is 1.530~1.686, and the birefringence index is not measurable for aggregates. No dispersion phenomenon. The hardness is 2.5~4.5, and the density of natural freshwater pearls is generally 2.66~2.78 g/cm³. The toughness is good, but it varies depending on the origins.

The internal structure of pearls is a layered structure, similar to the annual rings of trees (Figure 4). We can determine the growth period and age of pearls based on the pearl layer.

Figure 4 Cross sectional view of pearls

The pearl mussel interacts with natural light to influence its degree of pearl formation and color, forming a typical, soft, and rainbow colored "pearl" luster. The unique luster of pearls depends on the reflection, refraction, and diffraction of light from the innermost structure of the shell - the pearl layer. The most precious pearls have a metallic, highly reflective luster. Pearl layer refers to the protein called conchiolin filled in the gap between calcium carbonate layers, and its structural relationship is similar to that of bricks and cement for wall building (Figure 5). The thinner and more the pearl layer is, the better its luster looks.

Figure 5 Electron microscope image of the fracture surface of the pearl layer

Due to the non-homogeneous nature of pearls with white streaks on their surface, their colors are mainly white, with pink, silver, cream, brown, green, blue, black, yellow, orange, red, gold, purple, and rainbow colors available; Pearls (especially artificially cultured freshwater pearls) can also be dyed yellow, green, blue, brown, pink, purple, or black (Figure 6).

Figure 6 Black pearl

3 Physiological and Biochemical Characteristics of Pearls

Pearl is a substance formed in pearl shells and mussels, and is a product of their metabolism. Its chemical composition contains not only a large amount of inorganic components, but also a certain amount of organic components and water. The inorganic component is mainly aragonite type calcium carbonate crystal (aragonite and calcite), with a small amount of magnesium carbonate (magnesite), accounting for about 91%. The organic component is mainly shell keratin, accounting for less than 5% of the total amount of pearls. The water content ranges from 0.6% to 0.8%. At the same time, there are various trace elements such as iron, copper, cobalt, silver, zinc, magnesium, molybdenum, nickel, and so on.

3.1 Inorganic components of pearls

The main inorganic components of pearls are calcium carbonate and a small amount of magnesium carbonate. This is because soluble mucus and insoluble mucus can be secreted from the mantle of mollusks to induce crystallization into aragonite or calcite.

In addition, the composition of pearls also contains 10 trace elements, including Cu, Fe, Zn, Mn, Mg, Cr, Sr, Pb, Na, K, Ti, V, Al, Ag, Co, etc. And trace elements can affect the quality and color of pearls. Like other gemstones, trace elements play an important role in the color of pearls.

By using a plasma emission spectrometer, the content of metal ions in freshwater pearl powder of different colors was analyzed. The results showed that the content of elements such as Zn, Ti, V, Ag, Mg, Sr increased with the darker color of the pearl, and its luster also increased. This indicates that the content of metal ions is positively correlated with the color and luster of the pearl. Peach red pearls mainly contain Mn, and also contain more Mg, Na, Zn, Si, Ti; Gold and cream colored pearls contain metallic components such as Cu and Ag; Silver pearls contain Mn, Na, and Ti; Golden pearls contain the most types of metals; Silver gray pearls contain more organic matter, while white pearls contain less organic matter.

3.2 Organic components of pearls

The main organic components of pearls are shell keratin (also known as keratin or solid protein) and various pigments.

The mass fraction of organic components in pearls accounts for approximately 3.5% -7%. Extract shell keratin from freshwater cultured pearls, hydrolyze it, and use an automatic amino acid analyzer to detect various amino acids such as aspartic acid, serine, threonine, and glutamic acid (Table 1). In addition, the color of pearls varies due to the influence of various organic pigments (Zhang, 2006). The amino acid content of pearls with different colors varies slightly.

Table 1 Types and content of amino acids in freshwater cultivated pearls Note: Unit: wB/%

The chemical composition of different types of mother oysters varies greatly, resulting in significant differences in the nutritional and medicinal value of pearls. Freshwater pearls have lower value than seawater pearls.

On the basis of studying the chemical composition of pearls, people have also delved into the phase composition of pearls. There are currently two main viewpoints. One view is that pearls are mainly composed of calcite, and the other view is that pearls are mainly composed of aragonite, and the content of aragonite directly affects the quality of pearls.

4 Conclusion and Outlook

Pearl has been loved by the royal family since its birth due to its soft luster and warm texture, and is also known as the "one queen" of the "five kings and one queen" in the jewelry industry due to its unique properties. Pearls with rich colors and diverse shapes have extremely high ornamental value, research value, and health benefits. With the gradual rise and development of the pearl industry, pearls have attracted more and more attention from domestic and foreign consumers and researchers. Due to environmental and technological limitations, the collection of natural pearls is still relatively scarce. Artificial technology and scientific breeding of pearls are currently the main force in the pearl market. Environmental factors in different regions lead to significant differences in pearls. Therefore, studying the origin, structure, appearance characteristics, and physiological and biochemical characteristics of pearls can help us better understand their mechanisms, cultivate pearl varieties, and comprehensively promote the development of the pearl industry.

Pearls are divided into two types: freshwater pearls and seawater pearls. Due to geographical location and natural conditions, China mainly produces freshwater pearls. Nowadays, China has become the world's most important producer of freshwater pearls. The main freshwater pearl breeding areas in China are in Zhuji, Changde, Suzhou, Jiangxi, Hubei, Anhui, and other places. As a primary industry, pearl farming contributes to promoting local economic development and solving labor employment problems. With the development of policies, pearl cultivation, pearl breeding, and pearl sales have become iconic industries in many regions. How to better and comprehensively develop the pearl industry into a leading industry has become a key point in regional economic transformation. Therefore, we need to delve deeper into the causes of pearl formation and the conditions that affect its growth, and conduct controlled experiments to develop more scientific, efficient, and high-quality pearl farming.

The interaction between pearl shells, mussels, and natural light, as well as the environment in which pearl shells and mussels live, are often related to the color and shape of pearls. These two factors play a crucial role in the production and texture of pearls. The presence or absence of artificial intervention and the type of water environment in which pearls grow are also closely related to their final appearance. In this regard, there is relatively little research on pearls, so it is necessary to strengthen research on these aspects in the future, which is of great significance for further understanding the formation of pearls and has important practical application value for the research and development of new pearl varieties in China.

Authors’ contributions

Z.D. was the main author of the review, completing the collection and analysis of relevant literature and writing the first draft of the paper, and participating in the analysis and organization of literature and materials; G.H. participated in the analysis and organization of literature and materials; J.L.F. was the conceiver and person in charge of the study, directing paper writing. All authors read and approved the final manuscript.

Acknowledgments

This research was funded by "Cuixi Innovation Research and Development Project Fund" of Zhuji Cuixi Academy of Biotechnology.

References

Farn, and Alexander E., 2013, Pearls: Natural, Cultured and Imitation, Burlington: Elsevier Science, pp. 90–108. ISBN 9781483162737.

Zhang B.L., 2006, Systematic gemmology, Geology Press, Beijing, China, pp.102.

Sha N.L., and Zhang X.H., Pearls,Geology Press, Beijing, China, pp.38-48.