At a Polynesian coral lagoon in the 1930s, divers descend eight meters (25 feet) underwater to collect black-lipped pearl oysters (Pinctada margaritifera). Only about one in 10,000 of these oysters produces a gem-quality pearl, but every shell is valuable. Inside, these shells are coated with iridescent nacre or "mother of pearl"—the same substance from which pearls are formed.

Pearl diving can be difficult and dangerous. Repeated dives into deep water place great stress on the body because of the varying pressures at different depths. And although most sea life presents little danger to humans, sharks can be a hazard, and reaching into a crevice can occasionally result in a bite from a startled octopus or moray eel.

Making pearls
Pearls and shells are made by similar processes. Every mollusk that can make a shell can also make a pearl of some kind.

Single-shelled mollusks, including abalones and other snails, can make pearls, but the best pearl producers are bivalves: two-shelled mollusks such as oysters and mussels. These animals pump water through their bodies to filter out food, increasing the chance that a small particle or organism will get stuck inside. Pearls form when a mollusk encases these tiny invaders with layers of calcium carbonate.

Harvesting Pearls Newly harvested freshwater pearls are washed—here, in China—and then sorted by size and graded for quality. Both marine and freshwater mollusks can produce gem-quality pearls. © Paula Mikkelsen / AMNH

Today, instead of waiting for pearls to occur by chance, pearl farmers insert small spheres into pearl oysters so that pearls will form around them. Containers of pearl oysters are returned into the water and retrieved when the pearls are ready to be collected at least two years later. Cultured pearls are real pearls; the only difference is that the nucleus is introduced by humans instead of by chance.

ADAPTATION: Humans in Water
Most marine organisms can extract dissolved oxygen from water. Humans, however, can breathe only air, so divers must hold their breath while underwater or bring air with them. Humans are also poorly adapted to the high pressures created by many meters of water weighing down on them. Rising too quickly can cause nitrogen gas bubbles to form in the tissues, a deadly condition known as decompression sickness or "the bends."

Humans must also work hard to force their buoyant bodies underwater. Pearl divers traditionally carried heavy weights to help them sink as deep as 60 meters (200 feet). In contrast, many sea animals weigh about the same as the water around them, and some can adjust their buoyancy by inflating or deflating an internal gas bladder, which helps them rise, sink, or hover in place with little effort.

Cultured Pearls, Past and Present
The practice of inducing mollusks to make pearls goes back more than 1,500 years.

As early as the 5th century, the Chinese produced pearls by inserting small objects, including images of Buddha, between the shell of a freshwater mussel and its nacre-producing mantle. These "blister pearls" were raised shapes attached to the shell. In the 1750s, the Swedish naturalist Carl von Linné, also known as Linnaeus, created the first cultured round pearls by spacing the inserted objects away from the shell with a silver wire.

Modern pearl culturing blossomed in Japan under the leadership of Kokichi Mikimoto. He harvested his first blister pearl in 1893, and by the 1930s he had transformed the pearl industry. By inserting round pieces of freshwater mussel shell into the flesh of akoya pearl oysters (Pinctada fucata), his workers produced perfectly round pearls by the millions.