Oyster pearls are pearls that are made by marine oysters and freshwater mussels as a natural defense against an irritant such as a parasite entering their shell or damage to their fragile body. When an irritant, such as a food particle or a parasite, slips between the shells of an oyster or other mollusk and lodges into its mantle, the muscular wall where its internal organs are located, the mollusk will coat the offender in nacre-producing cells to form what’s known as a pearl sac. These cells wrap the threat in layers of proteins and aragonite until eventually, the cocoon completely absorbs the invader. Nacre can form naturally around almost any irritant that gets inside the shell, creating some very unique and precious pearls. When pearls are cultured commercially, an irritant is manually inserted into a mollusk to promote the production of mother-of-pearl.