The four Mollusks
It has been estimated by scientists that only about a few dozen of mollusk species produce pearls and about half of them are used to produce cultured pearls.
There are two types of mollusks: saltwater and freshwater. Pearl-producing mollusks that live in salt water are often referred to as oysters. Pearl-producing mollusks that live in fresh water may be referred to as mussels.
When a natural irritant such as a fragment of shell, a scale or a parasite becomes lodged inside an oyster or mollusk, it gets coated with layer upon layer of nacre. As the layers of nacre build, the pearl grows. Depending on the type of mollusk, it can take from six months to seven years for the pearl to form.
There are four species of Mollusks that are primarily used to grow cultured pearls: akoya, Tahitian, South Sea, and freshwater.
The pearling regions of the South Seas stretch between the southern coast of Southeast Asia and the northern coast of Australia, and extend up to the Philippines. Australia is the largest producer of South Sea cultured pearls.
The saltwater Pinctada margaritifera or black-lipped oyster is from the Polynesian islands. These oysters produce the worlds second most valuable pearls "Tahitian Pearls" or "black pearls". Contrary to their name, Tahitian pearls are not actually cultured in Tahiti. These pearls are grown in the tropical lagoons and atolls located in the French Polynesian island chain by small family or community-owned farms. The harvests are sent to Tahiti for export.
The dark edge of the shell and its large size confirm that this is the shell of the Black-lipped Pearl Oyster. These oysters are the only ones that can produce pearls with a full color spectrum. Tahitian pearls are especially known for colors like dark grey, green, blue, purple and peacock. .
Akoya cultured pearls are often perfectly round and have a high luster. Their color range varies from white to cream-colored and light pink. Akoya oysters are found on areas of the eastern coastline of North and South America, the east-coast of Africa, the Mediterranean and throughout the Indo-Pacific. Most notably, the Akoya oyster is found in Japan, where it has formed the basis of a multi-million dollar pearling industry.
The freshwater pearl mussel is an endangered mollusc species that is found mainly in clean, nutrient-poor, low-calcium rivers, ponds and lakes in China
Freshwater pearls used to be perceived as inferior compared to saltwater pearls, but today that has changed, and freshwater pearls have increased in quality and popularity. Freshwater pearls also have a very wide range of beautiful colors and different shapes.
Image sources: https://www.thoughtco.com/how-do-pearls-form-2291787 1.8, https://www.belpearl.ca/pages/pearl-types 1.8.