Some parasites or sand occasionally enter the shells of shellfish such as clams and mussels. In the innermost layer of each pearl shell or clam, there is a shiny mantle called "pearl layer". Constantly secrete nacre, and wrap the invading sand and parasites layer by layer. After a long time, the nacre wrapped by sand or parasites becomes pearls. For shellfish, this is an act of defending their bodies.

Reasons for the formation of natural pearls:

1. When mussels eat at the bottom of the sea, their shells will open and foreign bodies such as sand and parasites will occasionally fall in. When the mantle is stimulated, it secretes nacre, which wraps the falling foreign body layer by layer, making it smooth and gradually forming a pearl sac-a bright and moist outer layer.

According to this principle, cultured pearls are formed by inserting round beads into mussels by nuclear insertion technology.

It is generally believed that the epidermal cells of the mantle of mussels rapidly divide and proliferate under certain stimulation, gradually surround the stimulation source, form a complete pearl sac, and gradually secrete nacre around the stimulation source, thus forming pearls. Therefore, whether the pearl formed is spherical or non-spherical depends on whether pearlite can accumulate evenly around the stimulus.

There are two reasons for the formation of natural pearls:

1. Foreign bodies such as sand, worms or wormholes occasionally invade the shell of mussels and fall into the connective tissue of mussels together with some epidermal cells in the mantle. Epidermal cells divide and proliferate into pearl sacs, surround foreign bodies, secrete nacre, and finally form pearls. The pearls thus formed are nucleated.

2. Due to pathological changes or injuries, some epidermal cells of the mantle leave the original site, enter the connective tissue, divide and proliferate to form a pearl sac, thus forming pearls. The pearls thus formed are seedless.

▲ Features

The inner layers of pearls and mussels are the same. Its shape is changeable and can be divided into regular shape and irregular shape. Regular shapes are round, semicircle, steamed bread, oval, rectangle, rod, oblate, pear and so on. The shape of irregular nucleated beads varies with the shape of the nucleus. All seedless beads are irregular except those that can be described as regular.

The hardness of pearls is 3.5-4.5, which is higher than that of copper (3) and similar to that of iron (4).

The specific gravity of freshwater pearls is about 2.70, the specific gravity of high-quality pearls is slightly larger than 2.65-2.68, and the specific gravity of inferior pearls is relatively small.

The elasticity of pearls depends on the chitin content. Let a pearl fall from a height of 70 cm and let it fall freely on the glass plate. Generally, the bouncing height of pearls is about 37 cm. And the high-quality pearls have a higher rebound height.

▲ Advantages and disadvantages

The quality, color and luster of pearls are the most important signs.

1. On the one hand, the color of pearls is produced by light reflection and light interference on the surface and inside of pearls, and it is also related to the body color of various pigments and metal elements contained in pearls.

2. Through spectral analysis, it is found that golden yellow and cream pearls contain more copper and silver; Fleshy and pink pearls contain more sodium and zinc; White and pink pearls contain more manganese. Therefore, the color of pearls can be divided into white, pink, yellow and blue. The color of a round pearl is emitted from a sphere, and its color varies not only with the type, intensity and background of the light source, but also with the light source and observation angle.