FridayAFM - Curse of the fake pearl

Héctor here, your AFM expert at Nanosurf calling out for people to share their Friday afternoon experiments. Today I bring you jewelry.

Some time ago I showed you what the shell of mother-of-pearl looks like, and I also imaged some buttons to see if they were really made of mother-of-pearl or plastic.

Long story short, mother-of-pearl shell is made of nacre which consists in tiles (a.k.a. aragonite tablets) that arrange in layers. I saw these layers when imaging in the mother-of-pearl and all the buttons... so I guess all the buttons where "real".

You can see that fridayAFM here.

1 FridayAFM from 2022 10 14.
Mother pearl.
Button A, phase image.
Mother pearl.

However, as nice and colorful as the shell is... the most famous thing is not the shell, it is the pearls. So, since I published that fridayAFM, I started looking for some pearls (on a budget), because I wanted to see if we can use an AFM to determine if the pearls are real or fake. A quick Google search tells me that the "household" methods involve acid, cutting the pearls, or using a lighter. Since all these methods are destructive (and we don't necessarily want to destroy the jewelry), checking what the AFM (non destructive) can tell, will be very interesting.

These are the pearls I got for this fridayAFM. Some of ebay, some from local charities, and in principle, none of them authenticated.

pearl 1 of 6-1
1 Pearls to be tested and how to mount them for AFM imaging.

On this occasion I will use the FlexAFM system, and a cork padded vise to hold the rings in place. Lucky for me, the system has enough room to place the vise and the rings under the AFM.

I start with the mother-of-pearl shell ring, because I think, due to its low price, it is unlikely to be fake.

pearl 2 of 6-1
1 Mother-of-pearl shell ring imaged in Dynamic mode with a Multi75 probe.

I have to admit that this one confused me a lot at the beginning, because I couldn't identify what I was looking at. My motto is that the best AFM images are taken when you already know 90% of what to expect and the AFM brings you the next 5% (the other 5% is still unknown, I don't want to be too ambitious). But in this case, knowing too much was misleading me as I was expecting something similar to the images I took of the mother-of-pearl shell. It turns out that this ring was cut at an angle and it is not the top layer of the mother-of-pearl shell, but almost like a cross section. Which by the way it is very cool, st it was something I wanted to do in the fridayAFM of the mother pearl and I couldn't figure out how to cut the shell and leave a flat surface to be imaged.

Next I went for the earrings and the pendant. They came from the same shop, and look like belonging to the same set, but they were sold separately and there is no guarantee they go together. By the way, from here on, I changed to Static force mode as I notice I was interacting with contamination and it was hard to keep it from becoming attached to the probe. (I mean, in Static force it doesn't matter too much if the probe gets contaminated as long as it is still sharp... but in Dynamic mode, it might modify the resonant frequency of the probe and then it messes up the imaging).

pearl 3 of 6-1
1 Images of a fake pearl earing taken in Static force mode with a Multi75 probe.

This one is what I was expecting. Since I put out the mother-of-pearl images, I had several discussions online, and I learnt that sometimes jewelry has protective coatings. So I was expecting a coating, the question that remains is if there is nacre under the coating or not. Lucky for me, the coating has enough holes to let me see through, otherwise I would have to use some solvent... In any case, through the holes I can see structures with terraces, but they are too small for them to be nacre, and there is no trace of the aragonite tablets anywhere. So, conclusión, a fake pearl. Which by the way, one could have told just by looking at the base of the earring. They melted the material to introduce the metal rod... and nacre doesn't melt like that... plastic does. 

Next one is the pendant. I no longer have high hopes ... but one never knows...

pearl 4 of 6-1
1 Images of a fake pearl pendant taken in Static force mode with a Multi75 probe.

Unfortunately, this is again the same type of structures as in the earrings, so fake again. However, it is very interesting the terraced structures seen through the holes. I cannot quite make up yet what these are (well, I do know now, but if I tell you, you wouldn't keep reading). In any case, a knife and a little bit of scratching of tears of the coating revealing the inside of the fake pearl.

Three pearls so far, three fake. Surely the one in the nice box has to be real. I was skeptical of grey pearls, but checking online it turns out that this brand is well known for the colored pearls. By the way, I cannot find this particular ring in their catalog, so maybe it is an old model?

pearl 5 of 6-1
1 Fake pearl ring imaged in static Force mode with a Multi75 probe.

It is not, its a fake. Somebody probably took a real box and stuck a fake ring in it, that is why the design looks similar to the rings of that brand but not quite. In any case, if there were doubts, I scratched it and it peeled of easily revealing the inside, which seems like plastic. By the way, inside plastic, and again these terraces... but thanks to a lucky find I know what they are. These are terraces in calcite, and according to Wikipedia, plastic, calcite and iridescent nylon are quite common materials used in fake pearls. So mystery solved. But hold on, did I get a real pearl in the end or not?

Lucky for me, the last one was real.

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1 Real pearl ring imaged in static force mode with a Multi75 probe.

The terraces were evident even in the optical microscope, and it is the sphere with most defects. By the way, I changed the color scheme to match the fridayAFM about mother-of-pearl.

So... in the end we did find a real pearl (which now I will treasure for a long time), and it is worth between £50 to £2000 according to Google... much more than the price I paid for it, but taking into account the time spent, the probes, and the AFM... it is probably not a good investment.

I hope you enjoyed what I showed you. Stay in touch, and if you have more info, please share.

If you made it this far, here is another fridayAFM on jewelry. In this case it is the silica sphere that produces the light interference to create the rainbow color effect. Note: from the discussions we had on LinkedIn and Twitter, I learnt that sometimes these rings are coated with a protective layer, so I was looking fur such layer in the pearl rings.

Opal ring
1 FridayAFM from 2023 04 21.





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