Pearl is the stone of the Moon (Chandra) in the Navratna, and unlike the other eight it was made by an animal — a mollusc laying down layers of nacre (aragonite and conchiolin) around an irritant. Two facts dominate its market. First, natural pearls — formed without human intervention — are so scarce today that essentially all commercial pearls are cultured, with the nucleus inserted by a farmer. Second, the difference between natural and cultured is invisible from outside and is decided by X-ray.
What pearl is
Concentric or irregular layers of nacre secreted by a mollusc. Properties: SG 2.60–2.85 (nacreous saltwater pearls), hardness 2.5–4.5, a characteristic overlapping-platelet surface visible at magnification, a "gritty" feel against the teeth from those platelets, and a soft pearly lustre (orient) from light interference in the layers. Refractive index is not measured on pearls in routine practice.
Natural versus cultured
Natural. The mollusc formed the pearl around a natural irritant. Internally the nacre layers are concentric from the centre, or the pearl has a small irregular natural core. Very rare in commerce; most "natural" pearls offered are cultured.
Bead-nucleated cultured (Akoya, South Sea, Tahitian, most freshwater of recent years). A round shell bead is implanted and the mollusc coats it with nacre. On X-ray, the bead shows as a large, uniform, sharply bounded sphere with a thin layer of nacre around it, often with a visible boundary and sometimes a small void or the organic tissue between bead and nacre. The nacre thickness — sometimes only a fraction of a millimetre in cheap Akoya — is measured from the image.
Tissue-nucleated cultured (traditional freshwater). A small piece of mantle tissue is implanted, no bead. On X-ray the pearl shows an irregular central cavity where the tissue was, and the structure can superficially resemble natural. Distinguishing tissue-nucleated freshwater from natural is one of the harder pearl determinations and rests on the shape and features of the central cavity, layer structure and, in some cases, advanced imaging.
Imitation pearls
Glass, plastic or shell beads coated with a lustrous "essence" (originally fish-scale extract, now synthetic). Detection: smooth rather than gritty against the teeth; a perfectly uniform surface under magnification with no platelet structure; coating visible at the drill hole, often flaking; SG wrong for the core material (glass heavier, plastic lighter than nacre); uniform size and colour in a strand that is too perfect. Shell-core imitations (Mallorca-type) are the most convincing and still show the coating at the hole. Imitation pearls and The pearl tooth test.
Treatments
Bleaching. Near-universal on Akoya and many freshwater pearls to even the colour. Considered standard; disclosed by careful laboratories.
Dyeing. Black, gold, pink and other colours are commonly induced. Dye concentrates at the drill hole and in surface blemishes; the colour is often too uniform and can be checked against natural colour by spectroscopy and UV.
Irradiation. Produces grey and black colours in freshwater pearls; detected by the colour of the nucleus and other signs.
Coating. Lacquer or polymer to improve lustre or protect thin nacre; detectable at the hole and by surface texture.
"Maeshori" and other lustre treatments. Chemical treatments to enhance lustre, not always detectable.
How the laboratory examines a pearl
Magnification for surface structure and drill hole. Weight, size and SG. UV fluorescence. X-ray radiography for nucleus, nacre thickness and internal structure — the determining test. Spectroscopy where colour origin is in question. Report states: natural or cultured (and type), nacre thickness where relevant, and treatments observed.
What a report can say
Cultured pearl, bead-nucleated, saltwater, bleached is a full statement for an Akoya. Cultured pearl, tissue-nucleated, freshwater, dyed for a coloured freshwater pearl. Natural pearl is a rare and valuable finding that a laboratory should issue only on X-ray evidence, and it may refer difficult cases for advanced imaging.
Frequently asked questions
Are cultured pearls real?
Yes — real nacre laid down by a real mollusc, with a human-inserted nucleus. They are not imitations. They are also not natural pearls, and the price difference is large.
Can a lab tell natural from cultured?
By X-ray, yes, for bead-nucleated pearls with certainty. Tissue-nucleated freshwater versus natural is harder and may need advanced imaging.
What is a "Basra pearl"?
A trade term for natural pearls historically from the Persian Gulf. Most pearls offered under the name are not; see "Basra pearl": what the name means.
Does the tooth test work?
It separates nacre from smooth coated imitations, and nothing more. See The pearl tooth test.
How should pearls be cared for?
Away from perfume, cosmetics, acids and heat; wiped after wear; stored separately from harder gems. Organic gems are soft.