In general, pearls are classified as saltwater cultured pearls or freshwater cultured pearls. They will be futher categorized according to color and shape. They are white or other fancy color. Their shape are round, off round, drop like, baroque or other irregular shape.
Saltwater pearls can be much more costly than freshwater pearls of similar quality and size. It is because the yield of the saltwater pearl is much less than that of the freshwater pearl and it takes longer time for an oyster to nurture pearls.

Most common pearl types are as belows:

Freshwater Pearl

Freshwater pearls cultured in Hyropsis Cumingi mussel, produced primarily in China, Japan and some in North America. The leading countries in the production of freshwater pearls are the United States, China, and Japan. China has become the leader in this pearl type by developing new freshwater culturing techniques. Nearly 96% of freshwater pearls today are produced in China with much higher quality, even making them comparable and indistinguishable from their saltwater cousins. The Japanese also have a rich history of freshwater pearl cultivation. Pearls from Lake Biwa are recognized worldwide as the quality standard.

Freshwater pearls are available in a wide range of colors, shapes, and luster. Colors include orange, lavender, purple, violet, blue, rose, and gray just to name a few. Freshwater pearls can be created from 3mm to 11mm in size. They are very difficult to find completely round or near-round. Only an average of about 2% of the all harvests becomes round. In addition to the round cultured pearl, which requires more sophisticated production techniques, freshwater pearl producers are also culturing pearls in special fancy shapes such as crosses, bars, and coins.

In addition to the versatility offered by the many color and shape options, the lower cost of freshwater pearls makes it an attractive value alternative to the saltwater pearl.

South Sea Pearl

South Sea pearls are saltwater pearls cultivated in the waters of Australia, Indonesia, the Philippines, Myanmar, Japan and Thailand. They produce 10-20 mm pearls in silver or gold color. South Sea pearls are also the most expensive pearl in the market, due to their rare size and color. No other pearls grow as large as South Sea pearls.

Due to the variety of the Pinctada Maxima oyster, these exceptionally rare and extraordinary South Sea pearls come in different lustrous colors. The silver lipped oyster produces pearls in the white, silver, aqua, and blue family of overtones, while the gold lipped oyster pearls exude cream, champagne, vanilla, and deeper golden tones. The natural colors of South Sea Pearls are truly rich and beautiful and no enhancement is needed to bring out their soft satin-like glow.

The legacy of the South Sea Pearl goes back thousands of years when early Australians used oyster shells, along with the pearls found in them, not only as decorations for their tribal costumes but also as currency to trade for food and tools. With the arrival of the European explorers in the 16th and 17th centuries, the South Sea Pearl became a precious global commodity. By the 1930s strict regulations were imposed to protect the industry and it was until the 1950s that South Sea Pearl farms began producing harvests of commercial value.

Akoya Pearl

An Akoya cultured pearl is produced by inserting a nucleus together with a tiny speck of mantle tissue into the Akoya Oyster (Pinctada fucata martensii). Mie, Ehime, Kumamoto, Oita and Nagasaki are the major culturing areas in Japan.
Akoya pearls are the most difficult to grow due to the low survival rates of the host oysters. Less than 5 in 10 will survive the nucleation process. Of the survivors, only about 40% will successfully encircle the shell nucleus irritant with nacre. Overall, less than 5% of pearl output can be considered "high quality".
The "high quality" Akoya pearls are more perfectly round than most other pearls and are known for their high luster and rich color. Unfortunately, for those who have a preference for larger pearls, they rarely exceed 10mm and they are exceptionally expensive. In addition to Japan, China is now a major producer of Akoya.

Tahitian Black Pearl

Tahitian Pearls are grown in the oyster called Pinctada margaritifera. This is, obviously, a pearl producing oyster, and is unique for its black lip, and its notably large size, with specimens commonly reaching a foot across and weighing as much as ten pounds. These saltwater oysters can grow as large as 12 inches in diameter and produce pearls typically ranging in sizes from 8 to 18mm. With the skillful efforts of Japanese cultivation experts, the first Tahitian pearls were cultured in the 1960s.

The only place in the world that this type of oyster can be found is in Polynesia (also known as Tahiti) and other Pacific Islands that lie nearby.

Though considered to be black pearls, Tahitian pearls actually come in a range of different colors include:

1. Black
2. Green
3. Purple
4. Grey
5. Cream
6. White
7. Peacock green (the most valued)

This makes the truly black pearls extremely rare, and quite valuable. If you should choose to buy a Tahitian pearl, you will find that they usually cost less than classic white South Sea pearl. A Tahitian pearl of satisfactory quality at about 9mm in size will usually cost around $200, while you could pay a few thousand dollars for 12-15mm pears of high quality

Other types of pearl:

Half spherical cultured pearls grown on the inner shell of the oyster referred to as Pteria Penguin, Mabe Pearls are primarily found in the tropical seas of Southeast Asia and in the Japanese islands around Okinawa. Mabe Pearls have a beautiful, rainbow colored iridescence that ranges from light pink, dark rose to slight bluish shades gleaming with a metallic luster. Having a flattened side, Mabe Pearls are an ideal choice for jewelry such as pendants, earrings and rings that allow for a secure setting and a concealed flat back.

The process of culturing Mabe Pearls, or half pearls as they are often referred to, begins with the gluing of a plastic nucleus to the inside of the shell. Once the hemispherical nucleus is covered with nacre, the pearl is carefully cut away from the inner shell, its bead is taken out, and the cavity filled in with a resinous paste and backed by a mother of pearl plate. Depending on the shape of the nucleus, unique shapes of half pearls can be achieved such as ovals, drops, and hearts.

Keshi pearl

Often referred to as seed pearls, Keshi Pearls are produced as a bi-product of the culturing process. When an oyster rejects its bead implant and particles of the mantle tissue used alongside the bead remains, this stimulates the production of nacre and an essentially all nacre Keshi Pearl is born. Keshi Pearls are quite small in size, come in a wide variety of shapes, and occur in virtually all shades of color.

Today these lustrous all nacre pearls are becoming exceptionally hard to find as South Sea Pearl farmers are improving their abilities and increasing their efforts to reduce the production of these Keshi Pearls. Since each oyster can only produce a limited amount of nacre, allowing a Keshi Pearl to grow leaves less for the cultured pearl being produced simultaneously within the same oyster. As the farmers x-ray each oyster to see whether or not the nucleus is still inside, they have the opportunity to re-nucleate and not allow the Keshi Pearl to develop.

Conch Pearl

Conch pearls are a rare and beautiful type of pearl which occur in the Caribbean from the sea snail called the Queen's Conch, or Stombus gigas. It is the rich and swirling array of colors that defines conch pearls from the rest, and makes it so unique and desirable.

The sensitivity and complicated spiral structure of the snail shell makes it impossible to reach the pearl forming area without damaging it. Every year no more than 3000 conch pearls are harvested and only 15 to 20% are suitable for making jewelry. Another factor that adds to the rarity of these valuable pearls is the increasing problem of worldwide environmental pollution that has lead to the drastic reduction in the number of Queen Conch. A ten-year fishing ban was recently enforced off the coasts of Colombia as an effort to protect these rare snails from extinction.

These distinctively unique and strikingly beautiful pearls are characterized by a wavy flame structure that gives the appearance of a fire burning on the surface. The Queen Conch produces pearls in a wide variety of enchanting colors: white, pink, cream, olive, brown, beige, yellow, orange, and red. The most valued of these pearl colors are red and pink, which oftentimes show a wavy structure resembling the finest silk.

Unlike other pearls, conch pearls are measured in carats with the average pearl weighing between 2 to 6 carats. A pearl size of 8 to 12 carats is rare and extremely high value.

Abalone Pearl

Abalone is a type of mollusk that is primitive, univalve (which means single-shelled), and at least thirty million years old. There are only eight varieties of abalone, which is low, when compared to other pearl producing mollusks such as oysters and mussels. Different forms of abalone are found anywhere from California to Alaska, as well as in Korea, Japan, New Zealand, and Australia.

Much like their Mother of Pearl shells, Abalone Pearls are treasured for their exquisite color and highly iridescent nacre. These pearls vary greatly in color including blue, green, magenta, pink, gold, bronze, gray, silver, cream, purple, and beautiful combinations of these multiple colors. The most rare and valued colors are rich magenta, peacock blue, and green. Their prices can be as high as $2000 per carat, in the case of the rarest, highest quality pearls.

Melo Pearl

Melo Pearls are produced by a very large marine snail called the Indian Volute, otherwise known as Melo Melo. These rare pearls are primarily harvested in Vietnam but also found in other Southeast Asian waters. Extremely large and generally very round in shape, their luster resembles that seen in conch pearls. Typical colors include yellow, orange, reddish, tan, and brown. All Melo pearls are natural pearls but no one is able to cultivate them today.