Technically the term pearl should only refer to a natural pearl. However, natural pearls are so rare that the term pearl is now used to describe cultured pearls these days. Pearls are classified as Saltwater Pearls or Freshwater Pearls.
Akoya Pearls: Saltwater (cultured) pearls from the Japanese Akoya oyster. Usually range from 2-9mm.
Biwa Pearls: Freshwater (cultured) pearls Cultivated in Lake Biwa, Japan’s largest lake. Lake Biwa was one of he fitst freshwater culturing sites and has been well known for it’s quality pearls.
South Sea Pearls: Generally it is any saltwater pearl found in the area extending from Burma and Indonesia down to Australia. South sea pearls range in size from 8-18mm.
Black Pearls: Naturally dark colored pearls. Not dyed!
Blister Pearls: Freshwater or saltwater cultured pearls that grow attached to the inner surface of the oyster or mussel shell.
Mabe Pearls: These are blister pearls that have been assembled by gluing a half bead nucleus to a shell. They are not as durable as solid blister pearls.
Mother of Pearl: This is the lining on the inside of the mullusk shell. It’s smooth and very economical when made into mother of pearl beads.
Seed Pearls: Very tiny, natural pearls that measure 2mm or less. That’s small!
Keshi Pearls: "Keshi" is the term for poppy seed in Japanese. These are accidentally grown pearls in the soft tissue of pearl mollusks. They are tiny pearls without nuclei that resemble poppy seeds, hence the name. Today the term "keshi" also refers to the bigger pearls without nuclei.