Pearls, whether natural or cultured, are formed inside mollusks. There are many species of mollusks that can produce pearls. But only a few of these species, whether in salt water or fresh water, are responsible for majority of today's pearls. 

These include three saltwater pearl mollusks:

Pinctada fucata, commonly called the akoya oyster. It's found in the coastal waters of Japan, China, Australia, and New Guinea, and is the species originally used in Japan for the first commercial cultured pearls.

Pinctada margaritifera, or black-lipped oyster. This species is found throughout the southern Pacific, particularly in French Polynesia. It produces Tahitian pearls, which are primarily black or gray.

Pinctada maxima, or silver-lipped and gold-lipped oysters. Found throughout much of the southern Pacific, this species produces the large white or golden color South Sea pearls. 

Freshwater mollusks, also called mussels, include:

Cristaria plicata, commonly called wrinkle shell or river shell. This mussel lives in the rivers and lakes of China, Vietnam, Japan, and Korea, and was the origin of the large volume of Chinese freshwater pearls produced during the 1970s and 1980s.

Hyriopsis cumingi, or triangle shell. This mussel is found in China.

Cristaria plicata and Hyriopsis cumingi mussels account for the majority of Chinese freshwater pearl production today.

Megalonaias nervosa, or washboard. Although several species from the Unionidae family have been used to culture pearls in North America, today virtually all are produced in M. nervosa. All pearl-producing mollusks build a mother-of-pearl-lining inside their shells. The substances that make up the pearl and the shell lining are the same materials. The iridescent lining on the inside of the shell is called mother-of-pearl, when it's on the pearl, it's called nacre.