The innermost layer of shellfish, such as clams on the seashore and pearls in fresh water, is the "nacre", which is composed of nacre secreted by the mantle. When parasites, sand and other foreign bodies enter clams and clams' shells, they cannot be discharged for a while. After being stimulated by itching, they will secrete nacre to surround them. Over time, these foreign bodies are covered with thick pearl layers and finally become bright pearls.

The pearls seen in the jewelry store today are basically cultured products. People inject a nucleus into a clam or clam, and it usually takes two years to collect pearls. This artificial propagation method was invented by Japanese Mikimoto by Yukio, and he obtained the patent right of this mode of production in 1896.

At first, Miki used a relatively small pearl mussel to produce pearls. The size of pearls is only 6-7 cm, so Japanese pearls with a diameter greater than 10 mm are very expensive. In recent decades, in the South Pacific and Indian Ocean, people have used relatively large pearl mussels to produce pearls, so pearls larger than 14 mm are not uncommon.

The largest pearl in the world, Laozi Pearl, weighs 6350 grams, almost as big as a watermelon. This rare treasure is now hidden in the vault of an American bank.

This pearl was bought from a tragedy in 1934. It is said that in the mountains of the Philippines, the son of a chief and his friends went swimming at the seaside. Suddenly, he heard the chief's son scream and then disappeared. People immediately jumped into the sea to look for him. When the chief's son was found at the bottom of the sea, he found that his right hand had been tightly clamped by a clam shell. After people cracked the giant shell with iron bars, they accidentally discovered this "king of pearls".