Nacre thickness, and the only pearl test that works

A cultured pearl is a bead of shell with a coating of nacre grown around it by an oyster. The bead is cheap. The coating is the pearl. Everything about quality, price and longevity comes down to how thick that coating is and how well the layers are ordered, and the second of those is visible immediately while the first is almost invisible and matters more.

The types, and what they really are

• Akoya: saltwater, Japan and China, 6 to 9 mm, round, the classic white strand with a sharp mirror lustre. Nacre is thin because the culture period is short, often under two years.
• Freshwater: mostly China, now often round and large, frequently tissue nucleated which means the whole pearl is nacre with no bead. Softer lustre, vastly cheaper, and improving fast.
• South Sea: Australia, Indonesia, Philippines, 9 to 20 mm, white to golden, long culture periods and thick nacre. A satiny glow rather than a mirror.
• Tahitian: French Polynesia, naturally dark, grey through green to aubergine, 8 to 16 mm, thick nacre.

Why thickness decides the life of the pearl

Nacre is aragonite platelets in a protein matrix. It is beautiful, it is also slightly porous and slightly soluble in acid. A thin coat, under about 0.4 mm, does two things over time: it loses its outer layers to the acid in skin and cosmetics, going dull, and it develops fine cracks as the protein dries, which is the crazing that ends in visible peeling at the drill hole.

A thick coat, half a millimetre and above, has enough depth to be cleaned, worn and polished by contact for decades. This is the difference between a strand that looks better at forty years old and one that looks tired at eight.

Testing in the shop

Four checks, in order of usefulness.

• The tooth test. Rub a pearl gently along the edge of a front tooth. Nacre is microscopically ridged and feels faintly gritty. Glass and plastic feel glassy. It is undignified and it is decisive.
• Look at the drill hole with a loupe. You can often see the layer boundary between nacre and bead, and you can always see whether the nacre is chipping at the edge of the hole.
• Roll the strand under a point source of light. A sharp, tight reflection means ordered layers and good lustre. A diffuse glow that never resolves means either a soft nacre or a thin one.
• Rub two pearls together lightly. Real pearls have a slight friction. Imitations slide.

The strand that looks brightest under shop lighting is often the one with the thinnest nacre. Surface polish flatters a thin coat.

Matching, which is most of the price of a strand

A necklace requires pearls that agree with one another in size, shape, colour, overtone and lustre, and the number of pearls that have to be sorted to find fifty that match is enormous. That sorting is the reason a strand costs several times the sum of its pearls, and it is genuine labour rather than margin.

Look along the strand at eye level rather than at individual pearls. Any pearl that pulls the eye is out of match, and one pearl out of match can be replaced, but a strand with three is a strand assembled quickly.

Care, which is unusually strict

Pearls go on last and come off first. Perfume, hairspray, hand cream and the acid in sweat all attack nacre, and the damage accumulates invisibly until it does not. Wipe them with a soft damp cloth after wearing. Never use an ultrasonic cleaner, never steam clean, never store in a sealed plastic bag because nacre needs a little humidity and will dry and craze without it.

Restring silk every year or two if the strand is worn often. The knots between pearls exist so that a break loses one pearl rather than fifty, and silk stretches, greys and eventually fails. It costs very little and it is the maintenance people skip until the afternoon they do not.

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