Pearl is produced by black lip pearl oyster or pearl oyster, Pinctada margaritifera ( var. cumingii) and P. vulgaris, which grow to a size of 12-15 cm after 3 years of culture. Pearl oysters occur throughout the Indo-Pacific Region. They have been fished for almost two centuries for the mother-of-pearl industry and fine pearls were occasionally harvested from the sea.
To overcome the decimation of wild populations, spat collection tests were undertaken at the beginning of the 20th century but the technique was only developed in the late 1950s. There was a renewal of interest in pearls in the 1960s and the first grafts were carried out by the French Fisheries Service, and the first grafted pearls were harvested in 1965. In the 1970s, through private initiatives, this oyster began being cultured for pearl production and a market was developed for Polynesian black pearls.
Hatchery rearing of spat is not yet fully controlled. Pearl oyster rearing chains are suspended from a long line. Spat for culturing is collected in the natural environment by setting artificial substrates. After 12-24 months without any intervention, the oysters grow to 5-10 cm and are then set on oyster-rearing chains suspended from the subsurface long lines at 6 to 10 m depth. 4,000 to 10,000 oysters are set on each of these 200 m oyster-rearing chains, which are spaced 10 m apart.
After 3-12 months, the oysters reach a size of around 10 cm and weigh about 120 g, and at this stage they are ready to be grafted. After grafting, the oysters are again set on long lines until the pearl grows, which is generally around 18 months. The regulations stipulate a maximum density of 2,400 oysters per 200 m oyster-rearing chain. Only 25-30% of these oysters form a marketable pearl, rest are rejected, die or produce poor quality pearls.
The kechi pearl is formed by inserting pieces of the mantle of one mollusc into the gonad of another. The result will be small, less than 7 mm, odd-shaped pearl but no nucleus is required and as many as 20 pearls may be produced. When a nucleus is used, larger than 7 mm pearls are formed, which are called “baroque”. Mabe pearls are grown extensively by the American pearl companies because they are relatively easy to implant. A semi-spherical core (plastic in the case of Cross and Peach, and mother-of-pearl in the case of Latendresse type) is inserted under the mantle, which is gradually covered by nacre resulting in a half pearl. Different shapes such as rounds, kechis, baroques and mabes can be produced from the same oyster by inserting nuclei of various shapes.
Harvesting natural pearls from the sea was expensive and difficult process that produced very few pearls. Hence pearls were extremely rare and extremely valuable. It wasn’t until the Japanese discovered how to nucleate molluscs to produce cultured pearls, that pearls became available to people other than the very wealthy.
Kokichi Mikimoto, the son of a noodle maker, worked with his dedicated wife Ume to develop a strategy that stimulated oysters to produce pearls on demand. It was this discovery that brought about the beginning of oyster farming and pearl culture. At the same time a government biologist, Tokichi Nishikawa, and a carpenter Tatsuhei Mise had also each discovered the cause of pearl formation independently.
With these discoveries, and Mise’s patent in 1907 for a grafting needle, cultured pearls became quite an important and competitive science. They didn’t even know about each other’s efforts until Nishikawa applied for a nucleating patent and found out that Mise had discovered exactly similar thing. They then collaborated and established the Mise-Nishikawa method, which still forms the base of cultured pearls today. Mikimoto further altered the Mise-Nishikawa method and created a technique to culture round pearls. This patent was granted in 1916 and then Mikimoto overshadowed the works of all others. For example, Mikimoto’s efforts were directly responsible for the development of the Akoya pearl.
SouthSea pearls grow in the waters off the coasts of Australia, the Philippines, Indonesia, Myanmar, Japan, and Thailand. In the 16th and 17th centuries when the European explorers arrived, the global demand for South Sea pearls increased and by the 18th and 19th centuries, the South Sea pearl producing oysters were brought to the brink of extinction. Culturing of pearls saved these oysters, and brought South Sea pearls to the level of 10% of the entire pearl market.
Tahitian pearls were originally believed to belong to God Oro who used rainbows to visit earth. When the French arrived, the pearls were cultivated in the French Polynesian waters. By the 1700, the number of traders and explorers increased so much that the pearl-producing molluscs quickly depleted and France was forced to take strict action and placed severe regulations on the pearl fishing off these islands. It wasn’t until the Japanese began the process of nucleation that Tahitian pearls could again be marketed.