Pearl

What it actually is

Nacre: microscopic platelets of aragonite, a calcium carbonate polymorph, bound by an organic protein called conchiolin and laid down in concentric layers. Hardness 2.5 to 4.

The lustre and the faint iridescent orient come from light interfering between those platelet layers, which is a structural effect rather than a pigment.

Being calcium carbonate, pearl is dissolved by acid. Vinegar, lemon juice, many cosmetics and prolonged skin contact with sweat all etch the surface, and once the nacre is etched it cannot be restored.

Natural pearls form when an irritant becomes lodged in a mollusc and the animal coats it. Cultured pearls form the same way around a deliberately introduced nucleus, usually a shell bead, and the nacre is identical.

Freshwater cultured pearls are often nucleated with mantle tissue rather than a bead and can be nearly solid nacre.

Where it comes from

Japan pioneered pearl culturing, with Kokichi Mikimoto producing the first commercially viable cultured pearls in the early twentieth century, and Akoya pearls remain the classic small round saltwater cultured type.

China dominates freshwater pearl production by volume.

Australia produces South Sea pearls, the largest cultured pearls, from the silver-lipped oyster.

French Polynesia produces Tahitian pearls, naturally dark, from the black-lipped oyster.

The Persian Gulf was the historic centre of natural pearl fishing, and Bahrain in particular built its economy on it until the culturing industry collapsed the natural pearl market in the 1930s.

Natural pearls today are rare enough that most are auction items with laboratory certification.

What the traditions claimed

Pliny called pearls the most valuable of all things, and the Roman appetite for them was substantial enough to worry moralists as a drain on the treasury. The story of Cleopatra dissolving a pearl in vinegar to win a wager comes from him, and while the economics are exaggerated the chemistry is sound: pearls do dissolve in acid.

In the navaratna, pearl is mukta, the gem of the Moon, prescribed for emotional steadiness and for the mother, and it is one of the gentler prescriptions in that system.

Chinese sources describe pearl in medicine and as an imperial material, and pearl powder has a long history in East Asian preparations.

In Christian symbolism the pearl of great price in Matthew's gospel made it an emblem of the kingdom of heaven and, later, of purity.

Pearl diving in the Gulf, in Japan by the ama divers, and in the Americas shaped whole coastal economies before culturing.

How it is worked with

Pearl is the patience stone, and the reason is not decorative: a cultured pearl takes years of an animal's life to form, and natural pearls form by accident over longer periods still.

The formation around an irritant is the reading most people reach for, and it is accurate rather than sentimental.

For care, the rule is simple and old: last thing on, first thing off. Put pearls on after perfume, hairspray, makeup and sunscreen, and take them off before washing, cleaning or exercising.

Wipe them with a soft damp cloth after wearing, store them flat rather than hanging so the silk does not stretch, and keep them away from other jewellery that will scratch them.

Restring a frequently worn strand every year or two. Silk stretches and the knots between pearls exist to stop the whole strand escaping if it breaks.

Buying it honestly

Almost all pearls on the market are cultured, which is honest, normal and not a defect. Natural pearls are rare and come with laboratory reports.

Imitation pearls are glass or plastic beads with a coating, historically made from fish scale essence. The tooth test still works: a real pearl, natural or cultured, feels slightly gritty against the edge of a tooth because of its microscopic layering, while an imitation feels smooth and glassy.

Under magnification, imitations show coating wear and a smooth surface at the drill hole where real nacre shows overlapping platelets.

Dyeing and irradiation are used to produce black, gold and coloured pearls, and must be disclosed; naturally dark Tahitian pearls are a different and more expensive thing.

Thin nacre over a large bead nucleus is a real quality issue in cheap Akoya pearls and shows as visible blinking at the surface.