Pretty little pearls — A wee primer on pearl cultivation
Posted on: July 09, 2020
Falstaff: “I will not lend thee a penny”
Pistol: “Why then the world's mine oyster/Which I with sword will open.”
In reality, when it comes to pearls, the world’s not just your oyster, but your bivalve. And since June’s the birth month of pearl, let’s explore how pearls form naturally and how they’re cultivated. First though, a little history.
Pearls are one of the few gems in the gem pantheon that is organic – almost all are inorganic minerals – and have been symbols of wealth and status for millennia. Historically, they were found in the Persian Gulf, the waters of Ceylon (Sri Lanka), and those of Venezuela and Panama. Freshwater pearls were harvested from Chinese rivers and lakes and European rivers.
In my last article on June’s birthstones I introduced you to the most famous natural pearl La Peregrina that was found in Panamanian waters and brought to Spain in the mid-16th century. For two hundred years it was owned by a succession of Spanish kings. Two paintings hanging in the Prado Museum by Diego Velázquez show Queen Isabel and Queen Margarita wearing La Peregrina while on horseback. Hmmm, not me. I leave the pearls at home when I saddle up Bella Rubina.
After Spain, ownership passed to Joseph Bonaparte (brother of Napoleon), then his nephew Napoleon III, King of France. Later, when exiled to England, the Emperor sold it to James Hamilton, Duke of Abercorn and it stayed in the Hamilton family until bought for Elizabeth Taylor in 1969. (Now there’s a story. Go back and read the last article for more dirt on that one).
La Peregrina was a natural pearl. But natural pearl harvesting is effectively finished. It began a sharp decline at the beginning of the 20th century with the discovery of oil in the Persian Gulf in 1908, rendering pearl sales a quaint revenue. Besides, pollution from offshore industrialization decimated the pearl beds. The decline of mother of pearl buttons in favour of mass-produced plastic ones in the 1920s was another blow to natural pearls. Mother of Pearl is made from the lining of the oyster shell; the harvested pearl was simply a bi-product.
While naturals still exist, the efforts to find them are up there with needles in haystacks. A diver could search hundreds of oysters to find a pearl, with no guarantee of any quality. Forget about amassing enough for a strand.
But the major reason natural pearl harvesting declined was the advancements in cultivation. By the 13th century freshwater blister pearl cultivation was already widespread in China. Blister pearls grow attached to the inside of the shell and must be cut away, with a piece of shell, to remove. Modern blister pearl cultivation began in 1890 in Japan and Australia paving the way to cultured loose pearls.
As Rhodes and Oppenheimer were to diamonds, the Japanese Kokichi Mikimoto was to cultured pearls: a name almost synonymous with the gem. It was Mikimoto who mastered full round pearl cultivation and by 1920 was selling his beauties internationally. A hundred years later it is still the name in cultured pearls. Pearl cultivation took a hit during WWII, but he quickly revived the industry and many American soldiers stationed in Japan brought pearls home for their sweethearts.
Cultivation soon spread beyond Japan. South Sea farms in western Australia began in the 1950s, a decade later the first Tahitian farms began off Manihi Atoll, French Polynesia and in the 1970s the Chinese began cultivating freshwater pearls.
Both natural and cultured saltwater pearls are grown in a variety of oysters. Freshwater pearls however are grown in mussels. Both of these are bivalves, a large class of molluscs, defined by dual hinged shells. ‘Bivalve’ is derived from the Latin bis meaning ‘two’ and valvae meaning ‘leaves of a door’.
The impetus for natural pearl growth in both salt and freshwater is similar: A foreign object (parasite or particle) gets lodged in the mantle (the soft tissue inside the mollusc), the mollusc reduces the irritation by coating the object with nacre, a composition mainly of platelets of aragonite (calcium carbonate) held together with conchiolin, a binding agent. Bricks and mortar.
Cultured pearls are similarly spurred on to form. Like the natural process, they require the planting of an irritant that the mollusc will coat with nacre. Nucleator technicians must be quick and precise to insert the irritant. Like any breeders, pearl cultivators care for their livestock. The molluscs are started in hatcheries, their health is monitored to ensure they have enough nutrients and oxygen, and their shells are periodically cleaned. Farms require clean water free from pollutants, harbours safe from harsh weather (sheltered bays, lagoons), and underwater environments typical of the mollusc’s natural habitat.
Freshwater mussels are implanted with small pieces of mantle tissue donated by another mollusc. Up to 40 nuclei can be inserted in one bivalve, resulting in the harvest of multiple pearls. Pearls grow quicker in warmer water, and as a result freshwater pearls are softer with a satiny luster.
Saltwater pearl cultivation also begins with the implantation of a piece of donor mantle tissue but it is accompanied by a tiny bead nucleus. The bead is made from the shell of North American or Chinese mussels. Together, the tissue and bead are implanted in the gonad of the oyster. The piece of mantle grows to form a sac around the bead and the nacre layers begin to build upon it. Typically, only one nucleus is inserted and the pearl grows more slowly in the colder waters resulting in a mirror-like luster.
Growing times vary but, generally, the more months the pearl is left to grow the thicker the nacre becomes. The nacre layers the pearl like an onion. Indeed, the name given to them in ancient Rome, Pliny tells us, is ‘unio’ from the term ‘unique’ because no two pearls are ever alike. ‘Onion’ has the same etymology and is similarly layered and unique.
Cultured whole pearls are categorized by whether they’re grown in salt or fresh water, the species of mollusc and the region of growth. This gives us the four major types of cultured pearls:
Akoya: Produced in Japan, China and Vietnam in the pinctada fucata oyster. Akoya means oyster in Japanese. Cultivation takes 6-24 months and yields the highest percentage of spherical pearls. The classic colour is white or with rose overtones but at Disegno I love designing with the harder-to-find, naturally coloured silver blues.
South Sea: As in the southern seas of Australia, Indonesia and the Philippines using the oyster Pinctada Maxima. The two varieties – gold-lipped and silver-lipped – account for the characteristic colours.
Tahitian: Primarily from French Polynesia and the Cook Islands and grown in the black-lipped Pinctada margaritifera oyster, these pearls are dark and iridescent, which gives rise to such varietal names as pistachio, aubergine, and peacock.
Freshwater: The majority of freshwater pearls are cultivated in Chinese lakes and irrigated ponds using the mussel Hyriopsis cumingi. About ten times more freshwater pearls are produced in China than all other producers combined. The annual production volume makes these pearls inexpensive.
Here’s a few other pearl terms you should know:
Mabe: Is an assembled cultured blister pearl. The domed blister pearl is cut from the shell, filled, and backed by mother-of-pearl.
Seed: Very small natural saltwater and freshwater pearls both round or irregularly shaped.
Keshi: Both salt- and freshwater, these are a by-product of culturing made from nacre-producing cells that broke loose from the implant. Small, baroque in shape, named for the Japanese word for poppy seed.
Abalone: Brightly coloured blister and whole pearls from the saltwater abalone shell, not a bivalve. It is more vulnerable and thus not readily cultivated.
The renown gem historian George Kunz wrote in 1913 that the pearl fishers of Borneo preserved every ninth pearl they found in a bottle with two grains of rice per pearl. This simple little procedure was understood to breed more pearls. No, wait. Not so simple: custom dictated the bottle needed to be stopped with the finger of a dead man. Small detail.