Types of Pearls
Pearls are classified by the type of water their host mollusc grows in, and by the region in which they are grown.
Freshwater pearls are known for their fascinating range of shapes and shades, with colours ranging from pure white to lilac, pink and peach.
They grow in mussels, and are cultured using a small piece of mantle, the tissue that secretes nacre, which is taken from another mussel and surgically implanted to stimulate nacre production. This tissue will eventually disappear as the pearl grows, creating a pearl of solid nacre. As many as twenty or more freshwater pearls can be grown in each mussel, which makes them much more affordable, however, freshwater pearls tend to have more irregular shapes, making large round freshwater pearls quite rare and expensive.
Freshwater pearls take between two and five years to grow. The longer they are left, the larger the pearls, but the greater the chance that they will become misshapen or fall foul of predators or disease.
Freshwater pearls are mainly farmed in Japan and China, especially along the Yangtsze River Delta in the Zhejiang Province of Eastern China. The US also produces freshwater pearls, but these are mainly for their own domestic market. Natural versions are found as far afield as Scotland and the Mississippi delta in America. Lake Biwa, near Kyoto in Japan, was once famous for its freshwater pearl production, and many people still use the term ‘Biwa pearls’ to describe unusually shaped freshwater pearls even though production there stopped in the 1980s due to pollution. Japan now has limited freshwater pearl production and focuses on saltwater pearls
Saltwater pearls grow in oysters, and are cultured using a small shell bead and a piece of mantle tissue. The shape of the bead helps to create a rounder pearl, however this bead remains at the centre of saltwater pearls, and so they are not solid nacre.
Only one or two saltwater pearls can be seeded in each oyster, and only one in five seeded oysters will produce a quality pearl. Saltwater pearls can grow in just six months to a year and a half, although many oysters die, either as part of the seeding process or when their pearls are harvested. This makes saltwater pearls much harder to cultivate, and therefore more expensive.
There are several main varieties of saltwater pearl, including Akoya, South Seas and Tahitian.
- Akoya pearls –the classic pearl, with a round shape, clean surface and a very high lustre with colourful overtones. These were the first pearls to be cultivated by Mikimoto in the early 1900s, using the Akoya oyster (Pinctada fucata), and many people use the terms Akoya and Japanese pearls interchangeably. Akoya pearls range from around 2mm to 11mm in size, with the small size of the oyster preventing them from growing any larger. Originally, Akoya pearls came only from Japan, although these days the smaller Akoyas, under 7mm, are more likely to come from China, in the Halong Bay region, and be processed in Japan.
- South Seas pearls – much larger and highly sought after pearls in white, cream or gold shades. These are cultivated across Australasia, from Thailand, Indonesia and the Philippines to the North Coast of Australia. South Seas pearls range from 8mm to 20mm in size thanks to their large host oyster (Pinctada maxima or silver/gold lipped oyster). As few as 10% of oysters will produce a round pearl, and this scarcity, combined with their large size, makes them highly prized. Indonesian and Filipino pearls tend to be golden or light yellow, while Australian pearls tend to be white or silvery.
- Tahitian pearls – some the most exotic and sought after pearls in the world, the naturally black or dark hued Tahitian pearls are not farmed in Tahiti itself but on the many islands of French Polynesia. The combination of a unique oyster (Pinctada margaritifera or black lipped oyster), together with the volcanic, mineral rich waters of the region, creates pearls of stunning colours from black to peacock green. Tahitian pearls range from 8mm to 18mm or more, and have an extra thick nacre layer of up to 0.8mm creating a high lustre which enhances their vivid colours. Less than 10% of Tahitian pearls are round, making these specimens especially prized.
Both freshwater and saltwater molluscs can create so-called Keshi pearls, which are pearls formed without an artificial nucleus. These are generally small, oddly shaped pearls that grow as a result of either the rejection of the implanted nucleus, or damage to the mollusc during the implantation process. The nacre that forms around the damage, or at the original site of the rejected nucleus, forms a pearl, just as it would in the wild in response to damage or an injury.
While their pearls are not the direct result of implanting a nucleus, they are still considered cultured pearls as they arise through culturing techniques on pearl farms.