Should you rub pearls against your teeth?

It’s difficult to get through June without thinking of pearls. Pearls are one of June’s birthstones, together with moonstone and Alexandrite (a form of colour change chrysoberyl). This is an adaptation of a post I made during my Gem-A Gemmology Foundation course in 2022 on pearls.

Last on, first off

Things I remember being discussed a lot were, can you tell the difference between an imitation pearl and a real natural or cultured pearl by rubbing it against your front tooth? And ‘last on, first off’. The alkaline nature of the nacre (the shimmering white layers of minerals that give pearls their characteristic sheen and colour) against more acidic skin can, over time, cause degradation. Sun screen, cosmetics and perfume also put pearls in danger. Wipe them down with a soft, dry cloth before you store them, and try not to store them in a massive knotted box of other harder, sharper jewellery.

Learning more about pearls continues to fascinate me as they are one of my favourite gems: part animal, part mineral. I am fortunate to have experience of natural saltwater pearls as family heirlooms that we had appraised a few years ago including an interesting lab report which confirmed they were naturally-formed saltwater pearls and not cultured (X-rays show a clear ‘bead’ in the centre of cultured pearls or the absence of one if natural). There is something exceptionally magical about feeling natural saltwater pearls against your skin. Natural saltwater and freshwater pearls (those not seeded by hand) tend to be more irregular in shape and perfectly spherical ones are incredibly rare. Sometimes people describe a natural pearl’s particular ‘glow’. Recently there has been an uptick in interest in natural saltwater pearls such as those from Bahrain brokered by Rapaport.

My home pearl lab

I thought I’d use this opportunity to take a closer look at my ‘everyday’ pearls to sort the imitations from the true pearls. I purchased these relatively inexpensively or found them amongst relatives’ jewellery boxes (with permission of course). My instruments were: 10x loupe, SWUV lamp (be very careful with SWUV and do not expose anything but the piece of jewellery or gem), LWUV torch, hands and teeth.

Fishy pearls

First, a 16″ strand of graduated pearls, spherical and regular with a golden-cream body colour and even orient. Orient is the name for describing the scattering and diffraction of light through the microscopic crystals of aragonite (calcium carbonate) that forms the nacre of the pearl.

I found the pearls in an old costume jewellery box (that should have been a clue but she said they came from her mother-in-law and she didn’t know what they might be). They looked so lovely to me! The heft (feeling of weight in the hand) and temperature were light and warm. On reading up more about imitation pearls, however, some alarm bells started to ring as I examined a selection of the pearls under the loupe, taking note of the texture, especially around the drill holes.

I also observed a kind of darker graininess that I read may also indicate imitation pearls of some sort. One of the end pearls showed a coating that had chipped away quite badly. Many of the drill holes showed irregular thickening of layers. I wonder if this is the kind of imitation pearl made by coating shell or plastic beads with guanine–a lacquer made from herring fish scales? To be honest I am so taken with the idea of fish scale laqueurs that I din’t feel disappointed to learn this was not a perfectly string Akoya pearl necklace (the Japanese cultured pearls that fetch premiums for their spherical perfection and evenness of tone and orient).

Hefty black pearls

Then I took a closer look at a black pearl necklace with pretty peacock iridescence. I always felt these were too heavy to be pearls, and they felt cooler than the others, but couldn’t really figure out what they were until I learned more about pearl simulants. Closer examination showed dimples in some of the beads and the coating on some looked almost like a lacquer and some streaks were visible. Very convincing from a distance but I suspect these are glass beads coated with a pearlescent paint, which would explain their feeling of coolness against the skin and weight compared with other pearls. The play of light is also not convincing against real pearls, here the light just spreads rather than diffracts into spectral colours.

Similarly I have a pink ‘seed pearl’ necklace or so I thought. Prior to examination I thought freshwater cultured pearls might have been dyed. The loupe is definitely my best friend. It was so clear to see that these were clear glass beads on bright pink string. Their lustre should have given them away – vitreous or glass like and not pearly – but it was easy and quick to see under 10x magnification.

Freshwater friends

Finally I took a closer look at three freshwater pearl items, two necklaces and one bracelet. One white freshwater pearl necklace is made from threaded elongated ‘off-round’ beads. These have been well-worn by me. I originally bought them about 15 years ago from the Natural History Museum in London, where they were sold as freshwater pearls. Hopefully a trustworthy source!

I could observe some of the concentric smoother rings around the drill holes although my regular wear had caused rubbing between beads, and barelling is also apparent: these are grooves that form usually at right angles to the drill hole and are caused by acidic wear. It also needs a good clean as there’s bits of wool from sweaters and clothes stuck between the pearls too.

The bracelet comprises irregular shaped freshwater pearls of various pastel hues, so assumed to be stained, a common treatment for freshwater pearls. The drill holes are also smooth and show concentric rings.

Fluorescence testing

These two items plus another freshwater double-strand necklace of rice-seed pearls all fluoresced a green colour under SW and LWUV whereas the imitation pearls did not.

Tooth test

While watching a gemstones.com video on gem tests on YouTube I heard about the ‘tooth test’. This isn’t a spurious ‘biting test’ but something more subtle.

Gently (VERY – and if not on your own jewellery you must get permission to do this) cradle the pearl between thumb and forefinger and slowly rub it against the biting edge of one of your incisors (front teeth). A real pearl (cultured or natural) will feel grainy and rough because of the microscopic vibrations caused by the tiny interlaced crystals that form the nacre against the enamel of your teeth. What you are feeling is actually the very subtle vibration in your jaw which is incredibly sensitive. The coatings used on imitation pearls, being lacquers or paints, will provide a ‘smooth’ sensation. After learning about this test I tested all the above pieces of jewellery and I can confirm the test works–however no test like this is fool-proof and should never be used as the only diagnostic to separate imitations. It will also not be able to indicate a natural or cultured origin.