Remember that the ‘tooth test’ only helps you distinguish between artificial and genuine pearls but won’t distinguish between natural and cultured. Other indicators are that the drill holes have edges which are free from chipping and that the pearl is not completely uniform. Natural pearls will have at least some surface imperfections visible with a loupe.
It’s a common myth that natural pearls form around pieces of sand or grit. It's theoretically possible, but a rare occurrence in the wild. Most natural pearls are formed as a defensive mechanism against parasitic worms and sponges boring into the shell. For cultured pearls, the producers transplant a speck of oyster mantle tissue into the host shell to provoke the reaction, or insert an artificially made spherical bead (which may itself be made from ground up oyster material). The latter also encourages more uniform shapes, but they usually only have a thin coating of nacreous 'pearl' material around them.
The only reliable way to determine if a pearl is cultured rather than natural is by X-ray but there are a few things you can check as indicators (at least on ‘white’ pearls), with varying degrees of reliability depending on how thick the nacreous layer is around the core.
- Cultured pearls are sometimes significantly less dense, although the composition of the artificial bead nucleus can be adjusted to have a similar density to natural pearls.
- It may be possible when using a loupe to examine the drill hole to see a difference in colour and texture between the nucleus and the external nacre in cultured pearls.
- If you roll the pearl (or strand) on a white surface under a strong light, thin-skinned cultured pearls will ‘wink’ at you, showing brighter and darker spots as you roll them. This is the bead showing through the nacre.
- If you hold the pearl over very bright light such as a powerful flashlight that allows the beam to be focussed, thick-skinned cultured pearls will usually show the interior bead as a darker round smudge and thin-skinned ones will reveal a series of parallel stripes representing the growth layers around the bead. You won’t see this in natural pearls.
- If you place the pearl against a black background (a small box lined with black velvet is perfect) and place it under a strong light you can usually see a thin brown line between the exterior nacre and the bead nucleus. Natural pearls are less transparent and will appear as a white, homogenous ball with no discernible inner rings.
