A cultured pearl is a real pearl that a mollusk grows after a person deliberately starts the process. A technician places a shell bead and a small square of mantle tissue, or tissue alone, inside an oyster or mussel, and the animal covers it with nacre over months or years. Almost every pearl sold today is cultured. The method was developed in Japan in the late 19th and early 20th centuries.
Early attempts
People tried to make pearls long before the modern method existed. Chinese farmers by the 13th century, and probably earlier, slipped small lead or tin figures of the Buddha between the shell and mantle of freshwater mussels. The mussel coated them with nacre, producing pearl-covered images fixed to the shell. These are blister pearls, attached to the inside of the shell, not loose round pearls.
In the 18th century the Swedish naturalist Carl Linnaeus cultured pearls in river mussels by inserting a small object on a wire through a hole drilled in the shell. He sold the rights to his method, but it never became a commercial industry.
The Japanese breakthrough
Three Japanese men are linked to the invention of the round cultured pearl. Kokichi Mikimoto, the son of a noodle maker from Toba, began trying to culture Akoya pearls in Ago Bay in the late 1880s with the help of government marine scientists. In 1893 he produced semi-round blister pearls, and he received a patent for that method in 1896. A red tide wiped out much of his stock along the way.
The step that made round pearls possible came from Tokichi Nishikawa, a marine biologist, and Tatsuhei Mise, a carpenter. Each worked out, around the start of the 20th century, that a piece of mantle tissue placed with a nucleus inside the oyster's body would grow into a pearl sac and coat the whole nucleus. Mikimoto took up the approach and received his own patent for round pearl culture in 1916. By the 1920s Japanese farms were producing round cultured pearls in growing numbers. The method used today is sometimes called the Mise-Nishikawa method.
Mikimoto was also the great promoter of the product. He opened shops abroad, showed his pearls at international exhibitions and defended them in court cases in Europe during the 1920s, where natural pearl dealers argued they were fakes. Tests showed the cultured pearls had real nacre, and they won broad acceptance. The Japanese pearl farming page covers Ago Bay and Mikimoto Pearl Island in more detail.
How bead nucleation works
Most saltwater cultured pearls are bead nucleated. The process runs roughly like this:
- Farmers raise young oysters, either collected from the wild as spat or bred in hatcheries, until they reach working size.
- Before the operation the oysters are rested or crowded so they relax and open slightly.
- A technician cuts small squares of mantle tissue from a sacrificed donor oyster. The quality of this donor tissue affects the color and luster of the finished pearl.
- Through a small incision, the technician places a polished bead and one tissue square into the gonad of the host oyster, with the tissue touching the bead.
- The oysters go back to the sea in nets or panels hung from rafts or longlines. Workers clean them regularly and move them as water temperature changes.
- The tissue grows into a pearl sac around the bead and starts laying down nacre. After the growing period, farmers open the oysters and harvest the pearls.
The standard bead is cut and polished from the thick shell of North American freshwater mussels, mostly from the Mississippi River basin. Shell beads have a density and thermal expansion close to nacre, so the finished pearl drills and wears well.
A healthy South Sea or Tahitian oyster may be nucleated again after its first harvest, with a larger bead placed in the existing pearl sac. Each round produces a bigger pearl.
Tissue nucleation
Freshwater mussels are usually tissue nucleated. A technician makes small cuts in the mantle and inserts only pieces of donor tissue, with no bead. Each mussel can carry a dozen or more grafts on each side of its mantle. Since the whole pearl is nacre, a tissue-nucleated freshwater pearl is solid nacre all the way through. The process is a large part of why Chinese pearl farming produces so many pearls.
Some Chinese farms now also bead nucleate freshwater mussels, producing large, round, bright pearls often sold under the trade name Edison. The Chinese farming page covers them.
Saltwater and freshwater methods compared
| Feature | Saltwater (Akoya, South Sea, Tahitian) | Freshwater |
|---|---|---|
| Host | Pinctada oysters | Mussels, mostly Hyriopsis species |
| Nucleus | Shell bead plus tissue | Usually tissue only |
| Pearls per animal | Usually one, sometimes two | Many per mussel |
| Typical growing period | About 8 months to 2 years or more | About 2 to 6 years |
| Interior | Bead with a nacre coat | Solid nacre |
Growing periods vary with species, farm and the size of pearl wanted. Akoya pearls in Japan were often harvested after one winter season, which gives the best surface but a thinner coat. South Sea and Tahitian pearls usually grow for around two years per round.
Keshi and other byproducts
Things do not always go to plan inside an oyster. Sometimes it rejects the bead but keeps the tissue, and the pearl sac grows a small, irregular pearl of solid nacre. Sometimes a stray piece of tissue grows a second pearl next to the beaded one. The trade calls these keshi pearls. They are cultured pearls, even though no bead is present, and labs work hard to separate them from natural pearls.
Mabe pearls are a related product. Farmers glue a half-round nucleus to the inside of the shell, the oyster coats it, and the dome is cut out, filled and backed with mother-of-pearl. Mabe pearls suit earrings and rings where only one side shows. The page on mother-of-pearl covers the shell material itself.
Nacre thickness and quality
On bead-nucleated pearls, the thickness of the nacre coat matters for both beauty and durability. A thin coat can look glassy, show the bead under strong light and wear through at the drill hole. Longer growing times and warmer water usually produce thicker nacre. Pearl grading covers how nacre quality fits with luster, surface, shape and size.
Many cultured pearls are also processed after harvest. Akoya pearls are commonly bleached and polished, and some are dyed or given a light pink tint. Irradiation can darken the bead of freshwater and Akoya pearls to produce gray colors. These steps should be disclosed, and the gemstone treatments page explains disclosure norms across gems.
How the industry spread
Japan dominated cultured pearl production for much of the 20th century. Pearl culture then spread through the Pacific and Indian Ocean regions. Australia, Indonesia and the Philippines developed the white and golden pearls covered on the South Sea pearl farming page. French Polynesia built its black pearl industry from the 1960s and 1970s onward. Chinese freshwater farms grew from the 1970s and later supplied most of the world's pearls by volume.
Frequently asked questions
Is a cultured pearl a fake pearl?
No. A mollusk grows every layer of a cultured pearl. An imitation pearl, such as a coated glass or plastic bead, has no nacre at all and comes from no animal.
Who invented cultured pearls?
Kokichi Mikimoto patented a method for blister pearls in 1896 and did the most to commercialize the product. The tissue graft technique for round pearls is credited to Tatsuhei Mise and Tokichi Nishikawa.
Can you tell a cultured pearl by looking at it?
Not reliably. Very round, well-matched strands are more likely cultured, but proof needs X-ray imaging in a lab.
How long does it take to grow a cultured pearl?
It depends on the type. Akoya pearls may be harvested within a year, South Sea and Tahitian pearls usually take about two years, and freshwater pearls often grow for several years.