Basically, all pearls can be listed in two categories: Freshwater Pearls and Saltwater Pearls. It can also be divided into Natural pearls and Cultured pearls. The high value of natural pearls doesn't mean they are better quality than cultured ones. That is just because they are rare, and their chances to be discovered by human can be even rare.
According to the pearl book of CIBJO, term “Pearl” should only refer to a natural pearl. And the term “Cultured Pearl” shall always be used for cultured pearl, e.g., Akoya cultured pearl, Freshwater cultured pearl. Nevertheless, cultured pearls are so common and natural ones so rare, and for the sake of brevity, we will use pearl as cultured pearl in this part.
the three latter ones are all saltwater pearls.
Other kinds of pearl:
Blister Pearl :Pearls that grow attached to the inner surface of the oyster or mussel shell.
Mabe Pearl: Assembled cultured blister pearl. When blister pearl is cut from the shell, the pearl nucleus is removed. The remained hole is filled with a wax and the open will be covered with mother-of-pearl. Mabe pearls are not so durable as blister pearls.
Half Pearl: Sawed whole pearl. This is to remove the blemish part of the whole pearl.
Keshi Pearls :that grow accidentally in the soft tissue or pearl culturing by-products after pearl harvest.
Scallop Pearl: Also called “lion's paw pearls”, are from scallop Nodipecten subnodosus.
Mother of Pearl :The hard pearly lining on the interior of a mollusk shell
Abalone Pearl: Pearls from Abalone. This kind of rare pearl usually has unique baroque shapes and fantastic rainbow luster. Their prices can be as high as $2000 per carat according to their quality.
Conch Pearl :Lovely pink pearls – come from the great Conch. Because of the rarity of the conch pearl, even small, pale and irregular ones can retail for over $500 per carat.