The mussel was hit by an external force. When the mussel body is impacted by external forces during its growth, such as manual handling and natural disasters, the pearl nucleus may fall directly from the rupture of the pearl sac due to vibration or extrusion. Pearls are jewelry or handicrafts made of mother-of-pearl in shells, but not all shells produce pearls. When the inside of a shell is stimulated or infected, it will secrete a substance to protect itself, which will gradually form a smooth sphere, that is, a pearl.