Pearl

A pearl is nacre deposited around something the mollusc could not expel. The animal is not making a jewel; it is walling off an irritant with the same material it lines its shell with, in concentric layers, because that is the only response it has.

What distinguishes a pearl from the shell lining is only geometry. The chemistry is identical — aragonite platelets in conchiolin — and so is the mechanism of its lustre. A pearl glows rather than glinting because light penetrates several translucent layers before reflecting, which is a different optical effect from the single-surface reflection of a polished stone, and it is why a pearl cannot be imitated convincingly by anything with a painted surface.

The grain of sand of the popular account is mostly wrong. Natural pearls usually form around a parasite or a fragment of the animal's own tissue; sand grains are expelled.

Cultured pearls are made by inserting something deliberately, and how it is done splits the market in two.

Saltwater culture — Akoya, South Sea, Tahitian — implants a spherical bead turned from freshwater mussel shell together with a small piece of mantle tissue from a donor oyster. The tissue grows a sac around the bead and coats it. The nacre layer is thin, from a few tenths of a millimetre up to two or so, over a bead that is most of the pearl's volume.

Freshwater culture, which is now overwhelmingly Chinese and is most pearls sold by number, implants mantle tissue alone with no bead. The result is solid nacre, which is why freshwater pearls are lighter, more variable in shape, and no longer the poor relation they were assumed to be for decades.

The difference is invisible from outside and obvious on an X-ray, which is exactly how laboratories tell them apart: a bead-nucleated pearl shows a sharp boundary between the bead and the nacre, and a tissue-nucleated one shows growth rings all the way in.

The oldest test is the best and still used by every dealer: draw the pearl gently across the biting edge of a tooth. Real nacre is faintly gritty because the surface is a landscape of overlapping aragonite platelets. Glass and plastic imitations are smooth. Coated imitations can pass this briefly and fail near the drill hole, where the coating chips.

Distinguishing natural from cultured is not a field test and has not been since the 1920s, when it destroyed the natural pearl trade almost overnight. It requires X-ray imaging of the internal structure.

And nearly every pearl on the market has been processed — bleached, and often 'maeshori' treated, sometimes dyed. Undyed black pearls come from one species in French Polynesia; a cheap black pearl has been dyed or irradiated.

Pearls were the most valuable gem in the world for most of recorded history, because they were the only one available without cutting technology and because they could not be produced at will. Roman sumptuary law tried to restrict them, the Gulf fisheries supported entire coastal economies for two thousand years, and La Peregrina passed through the Spanish crown to Mary Tudor to Elizabeth Taylor.

Kokichi Mikimoto produced a cultured hemispherical pearl in 1893 and a fully round one by 1905, patented in 1916. Within fifteen years the natural pearl market had collapsed — the Gulf fisheries with it, at almost exactly the moment oil was found there — and the price of a pearl fell by more than an order of magnitude.

It is the clearest case in the gem trade of a material losing its value not because it became less beautiful but because it stopped being scarce, and it is worth setting against every argument that a gem's price reflects anything intrinsic.

Pearl farming is unusual among extractive industries in being straightforwardly dependent on clean water: the molluscs are filter feeders, they will not lay good nacre in poor conditions, and a polluted or warmed bay produces nothing saleable. Farms therefore have a direct commercial interest in local water quality, and pearl oyster culture is sometimes cited as a net positive for the bays it occupies.

The qualifications are real. Farms are concentrated, and concentration brings disease — the Akoya mass mortalities in Japan in the 1990s cut production sharply. Freshwater culture at Chinese scale has caused eutrophication where feeding was used. And the historical natural fisheries were destructive in a way the farms are not: they were dived, the animal was killed to check, and the vast majority contained nothing.