A Look Inside Shows How Pearls Form in Oysters
Oysters continuously rotate a single, lustrous orb for at least a year before the pearl becomes jewelry.
There are new reasons to admire pearls, as a new study reveals the multi-step, time-consuming process required to produce a perfect pearl.
The study, published in the journal Royal Society Open Science, is the first to confirm speculation that a developing pearl continuously rotates as it forms during a 12–18 month period. The study further confirms other aspects of pearl formation, revealing how even the tiniest variations -- such as a minuscule bump on the surface of the pearl -- can lead to less desirable irregular shapes.
"Pearls are the only unique gems produced by a living organism," lead author Yannick Gueguen and his team wrote, adding that this dynamic environment can result in differences affecting both cultured pearls and those produced without human intervention.
Pearls formed without human intervention can be found in almost any shelled mollusk. Because pearls produced by oysters in the family Pteriidae are particularly hard and shiny, however, they are among the most coveted. These shellfish are referred to as "pearl oysters."
Gueguen, who is a researcher at the French Research Institute for Exploitation of the Sea (IFREMER), shared that the formation process of pearls, "either by natural means or through human intervention, is a response to an injury to the mantle tissue or a protection against an aggression or an intrusion by a foreign body." The "mantle" that he refers to is a layer of soft tissue that covers the body of an oyster, clam or other shellfish.
The shellfish aims to smooth over the foreign object so that it will not cause harm to the rest of the individual's soft body.
It takes just a small irritant to start pearl formation. The item can be as small as a grain of sand, or as large as a chipped-off piece of shell from another animal. The irritant may wind up in the oyster by accident, or intentionally as part of the cultured pearl process.
There are two basic types of cultured pearls: nucleated (beaded) and non-nucleated (non-beaded). Nucleated cultured pearls are produced by inserting a piece of mantle tissue from a donor mollusk and a nucleus, usually a spherical piece of shell, into the body of a recipient pearl oyster. Non-nucleated pearls are produced by grafting only a piece or pieces of mantle tissue, and no bead is inserted.
For the insertion, a small incision is made into the oyster's reproductive organ, which lies between the animal's digestive gland and the mantle. The bits of mantle tissue that are inserted serve as a catalyst, starting the creation of the pearl.
The catalyst leads to growth of a "pearl sac" -- a small bag -- around the irritant. This sac then secretes a substance known as nacre, which is a composite material. Most of the iridescent material that is seen on pearls, inside Nautilus shells and more, is nacre.
When the pearl sac secretes nacre, a form of calcium carbonate in the compound arranges itself in numerous hexagonal platelets. "Nature's ability to generate so amazingly complex structures like a pearl is impressive," the researchers wrote.
For the study, Gueguen and his team used a magnetometer to investigate what happens inside of oysters during pearl formation. The pearl oysters in this case came from Takarao Island in French Polynesia.
The monitoring equipment revealed "that a continuous movement of the nucleus inside the oyster starts after a minimum of 40 days" after the nucleus is inserted, the authors wrote. The rotation then continues non-stop for at least a year.
If organic matter gets on the nucleus, it can remain, causing grooves to form. Natural bumps on the surface might also appear.
The researchers additionally used high magnification to investigate what patterns appear on the surface of the developing pearls under different conditions. When bumps and grooves are present, the surface patterns look like irregular notches -- somewhat similar to a human fingerprint.
More rounded pearls have very precise spiral shapes on their surface. These shapes reveal "the growing steps of the nacre layer under the effect of the rotation," according to the scientists.
Oysters typically only produce one to two pearls at a time. The Akoya pearl oyster, shown here, is an exception, since it can produce up to five cultured pearls at a time. It then dies at harvest. South Sea and Tahitian pearl oysters, on the other hand, do not die at harvest and may live on to produce many more pearls.
Freshwater pearls undergo a different process altogether. When cultured, the initial mantle bits and irritants are not placed in the reproductive organ of the oyster, but instead are inserted into the mantle tissue itself. Up to 32 insertions can happen at a time, which helps to explain why these types of pearls are much less expensive than the larger, round ones.
In recent years, scientists have been experimenting with ways to produce unique pearls in other shellfish. Scientists from Florida Atlantic University's Harbor Branch Oceanographic Institute, for example, have figured out how to control cultured pearl production in the queen conch.
The queen conch is the largest of the six conch species found in the shallow seagrass beds of Florida, the Bahamas, Bermuda, the Caribbean Islands and the northern coasts of Central and South America.
At first, the queen conch was sensitive to traditional pearl seeding techniques. The spiral shape of its colorful shell also posed challenges, making it difficult for workers to insert the pearl formation materials into the conch. Specialized equipment, however, enabled the scientists to overcome these obstacles.
While white and black pearls are still highly coveted, newer types of cultured pearls, such as those from the queen conch (pictured), come in a wide variety and combination of colors. These include red, pink, orange, yellow and brown, in addition to the more traditional white.
The conch pearls look like they have a porcelain finish and exhibit incredible luster. Queen conches do not die after harvest and may live up to 40 years. If a jewelry aficionado can wait, this means that a single conch can produce enough pearls for a bracelet, earrings or some other item.
While the new study and prior research have solved many longstanding questions about pearl formation, some mysteries still remain. For example, the enzymatic and self-assembly mechanisms underlying pearl formation are still largely unknown to science.
The complex overall process, the beauty of the produced items, and the lingering mysteries about pearls, though, continue to fuel admiration of these precious gems.