How Do Pearls Form? A Defence That Turned Into a Jewel

The lustrous material is nacre, commonly called mother of pearl, and it is a composite of two things: microscopic flat crystals of aragonite, a form of calcium carbonate, stacked like bricks, bonded by thin layers of an organic protein called conchiolin acting as mortar. That brick and mortar arrangement is the reason nacre is far tougher than the mineral alone, since a crack cannot travel straight through and must deviate around each platelet, and it is studied intensively by materials scientists trying to reproduce the effect. The characteristic iridescence is not pigment but interference, since the platelets are close in thickness to the wavelengths of visible light, so light reflecting from successive layers interferes and produces shifting colour, the same mechanism that colours a soap bubble. Nacre is secreted by the mantle, the tissue lining the inside of the shell, which normally deposits it on the shell's interior and deposits it around an irritant when one becomes lodged.

The distinction that governs the entire trade is how the process was started:

• •A natural pearl forms without human involvement, usually around a parasite or a fragment of organic material rather than the traditional grain of sand, and is extremely rare, with historical estimates suggesting a tiny fraction of wild oysters contain one of any value
• •A cultured pearl forms after a technician surgically inserts a bead nucleus and a small piece of mantle tissue from a donor mollusc, which is what actually initiates nacre secretion
• •Saltwater cultured pearls, including akoya, South Sea and Tahitian, are usually bead nucleated, so a thin nacre layer surrounds a shell bead, and thickness of that layer is the main quality variable
• •Freshwater cultured pearls, produced overwhelmingly in China in mussels, are frequently tissue nucleated without a bead, so they are solid nacre and one mussel can produce many at once, which is why they are inexpensive
• •The process was developed commercially in Japan in the early twentieth century, principally by Kokichi Mikimoto and by others working in parallel, and it destroyed the natural pearl trade within a generation
• •Distinguishing the two now requires X-ray imaging to reveal the internal structure, since the surface of a good cultured pearl is indistinguishable by eye

Pearls are graded on several independent factors and the relative weight given to each varies by type. Lustre is regarded as the most important, meaning the sharpness of reflections on the surface, which depends on nacre quality and thickness and cannot be improved by treatment. Surface cleanliness counts blemishes. Shape matters, with perfectly round pearls the rarest in bead-nucleated production and baroque or irregular shapes far commoner and cheaper, though some irregular forms are prized. Size raises value sharply above certain thresholds. Colour includes both the body colour and overtone, with certain natural colours, including the greens and peacock shades of Tahitian pearls and the golden tones of some South Sea pearls, commanding premiums. Nacre thickness is critical in bead-nucleated pearls because a thin layer wears through, and a treated or dyed pearl is worth far less, which is why laboratory reports matter for anything expensive.

Pearl fishing was one of the oldest and most dangerous extractive industries, with free divers working the Persian Gulf, the Gulf of Mannar, the Red Sea and later Central America and Australia, descending repeatedly on a single breath with well-documented rates of injury and death, and with the trade organised in several places on coerced or indentured labour. Roman writers described extravagant sums paid for pearls, and they functioned as a primary store of portable wealth in several societies. The natural beds were substantially exhausted by overfishing by the early twentieth century, and culturing arrived just as they collapsed. Modern production depends on farmed molluscs and therefore on water quality, which makes the industry unusually exposed to pollution, warming and disease outbreaks, several of which have caused sharp production falls. Freshwater mussels in the wild are among the most threatened groups of animals globally, principally from river modification and pollution rather than from pearl harvesting, which matters because they perform substantial filtration in the rivers they inhabit.

A pearl is nacre, microscopic aragonite platelets bonded by protein, deposited around something the mollusc cannot remove, and its iridescence comes from light interfering between layers rather than from pigment. Natural pearls are extremely rare, and cultured pearls, started by inserting a bead and donor mantle tissue, replaced them commercially within a generation of being developed in Japan. Lustre and nacre thickness decide value, and natural beds were largely exhausted before culturing arrived.