Sand and other foreign objects stimulate the mantle of the river mussel, secrete a large amount of nacre, and wrap the foreign objects layer by layer to form a pearl.
The literary saying is:
When small particles of foreign matter from the outside accidentally enter the shell, the pearl oyster will feel uncomfortable. In order to eliminate the discomfort caused by the stimulation of this foreign body, it instinctively secretes nacre to wrap the hateful foreign body layer by layer. As time goes by, it forms lovely crystal clear pearls. The little clam turns pain into strength, gloriously creates unparalleled jewelry, and exchanges its sorrow for human joy