Nothing on the outside of a pearl says whether it formed by accident in a wild mollusc or was started by a farmer inserting a nucleus. The lustre, the surface, the shape and the colour overlap completely. The difference is inside — in whether there is an implanted bead or tissue at the centre — and the way to see inside a pearl without cutting it is X-ray radiography. For pearls, as for Rudraksha, the X-ray is the decisive test.
What the radiograph shows
Pearl nacre and a shell bead are both calcium carbonate, but they differ in structure and density, and the boundaries between them show on a well-exposed image.
Bead-nucleated cultured pearl. A large, perfectly round (or near-round), sharply bounded core — the shell bead — surrounded by a layer of nacre that is often thin. The bead frequently shows its own faint parallel banding (from the shell it was cut from). Between bead and nacre there may be a dark line or gap where the organic conchiolin layer sits, and sometimes a small void. This picture is unmistakable, and it accounts for Akoya, South Sea, Tahitian and most modern round freshwater pearls.
Tissue-nucleated cultured pearl. No bead. At the centre, an irregular cavity or a dark, twisted feature where the piece of mantle tissue was — often described as a "tissue void" — with nacre layers growing around it. Traditional Chinese freshwater pearls are of this type.
Natural pearl. Concentric nacre growth from a very small centre, sometimes a tiny irregular natural nucleus (a grain, a parasite, a particle of organic matter), with the layers continuing to the surface. Natural pearls may also show a small central cavity, which is the source of difficulty in separating them from tissue-nucleated cultured pearls.
Imitation. A solid glass or plastic core with a thin coating that barely registers; or a shell bead with no nacre layering; the internal structure is not a pearl's.
Nacre thickness
On a bead-nucleated pearl, the nacre layer's thickness is read directly from the image. Thin nacre — sometimes a few tenths of a millimetre on inexpensive Akoya — wears through in use, exposing the bead, and is a quality and durability factor. Reputable reports state nacre thickness, and some markets set minimums.
The difficult cases
Separating natural pearls from tissue-nucleated freshwater cultured pearls is the hardest determination in pearl testing. Both may show a small central cavity; the examiner looks at the shape of the cavity (tissue voids are often elongated, curved or irregular in a characteristic way), the growth structure around it, and the outer layering. Advanced laboratories add micro-CT scanning, which images the interior in three dimensions, and sometimes trace-element analysis (saltwater versus freshwater origin from manganese and strontium). A bench laboratory should report such cases with appropriate caution or refer them.
Other things the X-ray reveals
Drill-hole damage and filled holes; cracks in the nacre; multiple nuclei in a single pearl; a bead of the wrong material (some cheap cultured pearls use non-shell beads); and, in strands, whether all pearls are of the same type.
How the examination is done
The pearl or strand is placed on the detector and exposed. Strands are imaged in sections. The image is read for nucleus type, nacre thickness and structure, and the findings are combined with surface examination (overlapping-platelet structure at magnification confirms nacre), UV fluorescence and, for colour, spectroscopy. Pearl (Moti): natural, cultured and imitation covers the surface tests.
Frequently asked questions
Does X-ray damage a pearl?
No. The exposure is brief and leaves the pearl unchanged.
Can a strand be X-rayed without unstringing?
Yes. Strands are imaged in sections and each pearl read individually.
Can the X-ray tell saltwater from freshwater?
Not directly. Bead-nucleated pearls are mostly saltwater (and now some freshwater); tissue-nucleated are freshwater. Chemical analysis separates the environments where it matters.
What if a "natural" pearl shows a bead?
Then it is cultured, whatever it was sold as. This is the most common finding in pearls submitted as natural.
Is a thin-nacre pearl a fake?
No — it is a cultured pearl of low quality. It is genuine nacre over a genuine bead; the thinness is a durability and value issue, and should be disclosed.