The chief component of the nacre that constitutes the pearl is aragonite CaCO. 
Pearls are formed by a mollusk consisting of the same material (called nacre, or mother-of-pearl) as the mollusk's shell. It is a highly valued gemstone. 
The shell-secreting cells of the mollusk are located in the mantleof its body. When a foreign particle penetrates the mantle, the cells attach to the particle and build up more or less concentric layers of pearl around it. Irregularly shaped pearls called baroque pearls are those that have grown in muscular tissue. Pearls that grow adjacent to the shell are often flat on one side and are called blister pearls. 
Pearls are characterized by their translucence and lustre and by a delicate play of surface color called orient. The more perfect its shape (spherical or droplike) and the deeper its lustre, the greater its value. Only those pearls produced by mollusks whose shells are lined with mother-of-pearl (e.g., certain species of both saltwater oysters and freshwater clams) are really fine pearls. Pearls from other mollusks are reddish or whitish, porcellaneous, or lacking in pearly lustre. 
The surface of a pearl is rough to the touch. Pearls come in a wide range of sizes. Those weighing less than 1/4 grain (1 pearl grain = 50 milligrams = 1/4 carat) are called seed pearls. The largest naturally occurring pearls are the baroque pearls; one such pearl is known to have weighed 1,860 grains. 
Cultured pearl is natural but cultivated pearl produced by a mollusk after the intentional introduction of a foreign object inside the creature's shell. 

 

The discovery that pearls could be cultivated in freshwater mussels is said to have been made in 13th-century China, and the Chinese have been adept for hundreds of years at cultivating pearls by opening the mussel's shell and inserting into it small pellets of mud or tiny bosses of wood, bone, or metal and returning the mussel to its bed for about three years to await the maturation of a pearl formation. Cultured pearls of China have been almost exclusively blister pearls. 
The production of whole cultured pearls was perfected by the Japanese. The research that led to the establishment of the industry was started in the 1890s by Mikimoto Kokichi, who, after long experimentation, concluded that a very small mother-of-pearl bead introduced into the mollusk's tissue was the most successful stimulant to pearl production. Cultured pearls closely approximate natural pearls.