Real, cultured or plastic? Testing a strand of pearls

Three categories, and only one is a fake

Pearls divide three ways, and the middle category causes most of the confusion.

• Natural pearls formed in a wild mollusc with no human involvement. They are rare, almost always antique, and most of those in circulation were strung generations ago.
• Cultured pearls grew inside a living mollusc that was deliberately seeded with a shell bead or a piece of mantle tissue, then left in the water for months or years. The mollusc laid down nacre around the implant exactly as it would around a natural irritant. A cultured pearl is a real pearl, and calling one fake is simply wrong.
• Imitation pearls are beads of glass, plastic or shell coated in a pearlescent lacquer. Nothing alive was involved.

Almost every pearl sold today is cultured. So the practical question in front of most people is not natural against cultured – it is nacre against lacquer.

The tooth test, and what it is telling you

Nacre is built from overlapping platelets of aragonite laid down in microscopically thin layers. Drawn gently along the biting edge of a front tooth, that structure feels faintly gritty or sandy. A glass or plastic bead under smooth lacquer feels slick, like a polished button.

Do it lightly, on your own pearls, using the edge of the tooth rather than the flat, and wipe the pearl afterwards. Rubbing two pearls together gives the same information.

The limits matter. Better imitations are made with a textured coating precisely to defeat this test, and a heavily polished cultured pearl with a very fine surface can feel almost smooth. It is also silent on the question people think they are asking, because natural and cultured pearls are both nacre and both feel gritty. Treat a slick pearl as a reason to reach for a loupe, and a gritty one as encouragement rather than proof.

The drill hole is the most honest part of a pearl

A 10x loupe and a bright light at the drill hole will do more than every other home test combined.

On a bead-nucleated cultured pearl you can often see the structure in section: a translucent nacre layer over a chalky white bead, with a fine darker line between them. The thickness of that layer is the biggest quality question about the pearl, because a thin one wears through at the hole and along the sides of the strand.

On an imitation the coating behaves like paint, because it is paint. Look for a ragged or flaking edge at the hole, a place where the coating has lifted to show white plastic or clear glass beneath, and holes that look rounded or slightly melted rather than crisply drilled. Dye often shows here too, pooled in the opening.

Look along the strand as well. Where pearls have rubbed each other for years, an imitation shows bare bead and a real pearl shows thinned nacre.

Surface, overtone and orient

Pearls grow, so they are never quite uniform. Across a real strand you will find faint ridges, small blemishes, pinprick pits and minute differences in shape and diameter from pearl to pearl. A machine-made strand is identical bead to bead, and that perfection is itself the tell.

Two words are worth knowing. Overtone is the faint secondary colour floating over the body colour – rose, silver or green over cream – most easily seen at the edge of a pearl in diffuse daylight. Orient is the shifting iridescence produced when light diffracts through many layers of nacre, and it moves as you turn the pearl. Both come from layered structure, and a lacquered bead can only manage a flat, even shine that sits on the surface and stays put.

Weight, temperature and what never to try

Two quick checks, with their limits attached. A glass-based imitation is noticeably heavier than a nacre pearl of the same diameter and a plastic one is noticeably lighter, so a strand that feels wrong in the hand is worth examining. And real pearls feel cool on first touch and warm slowly against the skin, where plastic is already at room temperature. Glass is cool too, so temperature separates plastic from pearl rather than imitation from real.

What never to do: no flame, no knife, no acid, no bleach, no ultrasonic cleaner and no silver dip. Nacre is soft, roughly 2.5 to 4 on the Mohs scale, it is held together by an organic binder, and it dissolves in acid. Every destructive test described online destroys the thing you were trying to identify. Pearls are strung on silk, which perfume, hairspray and sweat rot steadily, so they go on last and come off first.

Akoya, freshwater, South Sea and Tahitian

Cultured pearls come from different molluscs in different waters, and the type sets both the look and the price.

• Akoya – saltwater, farmed in Japan and China, typically 6 to 9 millimetres, very round and sharply reflective, white or cream with a rose or silver overtone. Bead-nucleated, so the nacre layer is relatively thin.
• Freshwater – farmed in mussels, overwhelmingly in China, usually grown from mantle tissue alone and therefore solid nacre. A wider range of shape and of natural colour, including peach and lavender.
• South Sea – the largest, roughly 9 to 16 millimetres, from Australia, Indonesia and the Philippines, in white, silver or a natural golden yellow, with thick nacre and a soft satin lustre.
• Tahitian – naturally dark pearls from French Polynesia, in grey, green, aubergine and the mixed tones called peacock.

Dyed freshwater pearls are sold as dark pearls constantly. Natural dark colour is uneven and complex across a single pearl, where dye looks flat and gathers at the drill hole.

Read the strand, not only the pearls

Knots between the pearls are a quality sign rather than a decoration. A knot stops the pearls abrading each other and means a broken thread loses one pearl instead of fifty. Good strands are knotted between every pearl, with each knot tight against the drill hole. Cheap imitation strands are often unknotted, though some are knotted precisely to look the part.

The clasp is evidence too. A strand of good cultured pearls almost always ends in a gold or silver clasp, often hallmarked, because nobody finishes a serious strand in plated base metal. Photograph the clasp from both sides, as our guide to photographing jewellery sets out.

Silk stretches and grows dirty. A strand worn often is normally restrung every year or two, and one whose thread sags visibly should not be worn until it has been.

What would actually settle it

An imitation can usually be identified in a minute by any jeweller with a loupe. Natural against cultured is a different matter, and there is one reliable answer: X-ray radiography at a gem laboratory. The image shows internal structure – a solid bead under a nacre shell in one case, concentric growth rings around a tiny centre in the other – and nothing visible from the outside gives that away. Laboratories such as GIA also use X-ray fluorescence to separate freshwater from saltwater material and spectroscopy to detect dye.

From a photograph, Carat can identify a strand as pearl, report type, shape, lustre and condition as observations, and flag a coated bead where the images show the coating failing. What it returns is an estimate, and on the natural-or-cultured question the honest answer from an image is unknown. If a strand might be natural – inherited, undocumented, unusually fine – the laboratory report is the money worth spending.