Q&A: Pearlfacts For Not-Pearlologists

[yssie]

Hey all!!

So... We all know that getting information in the pearls world can be a challenge!! It's easy enough to find some basics, in blogs and on websites and from other fora, and there are research papers galore. But there's hardly anything in-between!!

Any interest in a PS Pearl Q&A thread to try to bridge the gap? I'm thinking this could become a reference thread where we accumulate technical information for non-experts. Somewhere PSers can come for thoughts, and answers, and pointers to the #PearlMysteries that have been haunting their showers... Without us all having to become engineers, biologist, physicists, or optics experts ourselves.


Rules to play:
Ask a question about pearls! How they look, how they behave, the industry. Or answer a question that someone else has asked. Or share your answer(s) to your own question(s) with the rest of us! Discussion about answers is welcome!!

1. One question per post. Please quote the post(s) you're responding to if you're answering a question or commenting on another response!
2. Cite your sources!! Blogs, articles, videos, websites, whatevers. Let the rest of us know where we can go for more information about whatever you've just said.
3. All opinions and answers are to the best of that participant's knowledge. Don't wager your life savings on anything you read.
4. Be nice. Discussions can get heated. But I won't hesitate to report outright a$$hattery from anyone, tradeperson or consumer.
5. No requests for authentication or "help me choose between these pearls" questions, please! And no discussions about superglue vs. epoxy either, that horse is a poltergeist by now.

 

[yssie]

I'll start!! Here are some questions that I've been nursing for a looooong time - and some of the information I've found...

 

[Daisys and Diamonds]

Great idea for a thread Yssie
i love them but just dont have the time or energy to devote to knowing every single in and out

 

[yssie]

What creates a pearl's body colour?

Ignoring iridescence! And also ignoring treatments.

- - - - - - - - - -

Both pigments in the nacre (pigment binds to the conchiolin protein matrix, not the aragonite) and the structure of the nacre itself.

The metabolism (quantity and quality of nacre secretion) of the pearl sac depends on the environment that the graft recipient/host oyster provides. The host oyster is responsible for the nacre structure of a pearl, which translates directly into pearl quality - size, luster, blemishing, nacre thickness, smoothness of skin. The donor oyster, on the other hand, is primarily responsible for a pearl's pigmentation.

A younger host (12-24 months), regardless of donor age
- Can provide a more nutritious and protective environment, which *in general* produces a higher-quality pearl
- Is more likely to die from the implantation procedure than a larger (older) oyster

An older/second cycle host, regardless of donor age
- Is larger, and can handle a larger nucleus, yielding a bigger pearl
- Is more likely to survive the implantation process
- But slower metabolism means less nacre production

Using a younger donor (12-24 months), regardless of recipient age
- Is more likely to yield a lustrous pearl
- If we're talking about Tahitians, is more likely to yield a green-bodied pearl

Using an older donor (>24 months), regardless of recipient age
- If we're talking about Tahitians, is more likely to yield a grey pearl

So that's pigmentation and, y'know, the oyster getting it's generic annual physical.


Then there's the the nacre structure component. The first paper I'll link is a seminal work that's been referenced a gazillion times - it investigates how nacre structure itself impacts body colour when there's no pigmentation involved.

In this pic you're looking at a cross-section of a bunch of aragonite tablets - imagine if you cut straight into a pearl from surface down to nucleus.


In this pic the bricks are the sides (depths) of those aragonite tablets. So to cut from surface down to nucleus, you'd cut top left to bottom right of the pic. And the little rectangle top left is a closeup of the space between two layers of aragonite tablet. The paper calls that space the "edge band", and the totality of the effect of all of those "edge bands" throughout the nacre all over the pearl is the "edge effect".


The theory proposed in this paper - which everyone seems to agree with, even if some seem somewhat reluctant - is that how clean the edges of the tiles are, and how regular the spacing of the tiles is, is what determines what the body colour of a pearl will be in the absence of any pigmentation. Unbroken tiles, clean tile edges, an organized matrix of tiles with regular spacing results in a more saturated body colour, with the colour itself being determined by how wide the edge band is. The cool part is that they've (successfully!) modelled the correlation between edge band thickness and pearl colour!! If the edge band is ~75nm wide you get a silver pearl. If the edge band is ~90nm wide you get a gold pearl. And if the tiles are broken up, misformed, have raggedy edges, aren't organized well - you get a less saturated (and less "metallic") body colour.



My understanding is that the impact of these edge effects is basically totally outstripped by presence of any pigmentation. So with a Tahitian pearl... The body colour is whatever colour the pearl's pigment is, and this edge effect has negligible impact on that body colour.

- - - - - - - - - -

Snow, Pring, Self, Losic, Shapter. (2004) The origin of the color of pearls in iridescence from nano-composite structures of the nacre
http://www.ruf.rice.edu/~rau/phys600/p1353.pdf

Ky, Demmer, Blay, Lo. (2017) Age-dependence of cultured pearl grade and color in the black-lipped pearl oyster Pinctada margaritifera.
https://www.researchgate.net/public...ck-lipped_pearl_oyster_Pinctada_margaritifera

Le Moullac, Schuck, Chabrier, Belliard, Lyonnard, Broustal, Soyez, Saulnier, Brahmi, Ky, Beliaeff. (2018 ) influence of temperature and pearl rotation on biomineralization in the pearl oyster, Pinctada margaritifera.
https://journals.biologists.com/jeb...nfluence-of-temperature-and-pearl-rotation-on

Blay, Planes, Ky. (2018 ) Optimal age of the donor graft tissue in relation to cultured pearl phenotypes in the mollusc, Pinctada margaritifera.
https://www.researchgate.net/public...notypes_in_the_mollusc_Pinctada_margaritifera

Ky, Pabic, Koua, Molinari, Nakasai, Devaux. (2015) Is pearl colour produced from Pinctada margaritifera predictable through shell phenotypes and rearing environments selections?
https://archimer.ifremer.fr/doc/00285/39637/38129.pdf

Mamangkey, Agatonovic, Southgate. (2010) Assessing Pearl Quality Using Reflectance UV-Vis Spectroscopy: Does the Same Donor Produce Consistent Pearl Quality?
https://www.researchgate.net/profil...t-Pearl-Quality.pdf?origin=publication_detail

 

[yssie]

What creates iridescence in a pearl?

Not the body colour, but the coloured halo that shifts as you rock the pearl and that almost always disappears in direct sunlight.

- - - - - - - - - -

The average nacre layer, which is one layer of aragonite tablets plus one conchiolin binder layer, is about 0.5µm deep. And the refractive index of aragonite is about 1.6, and the refractive index of conchiolin is about 1.3.

So there are apparently two primary components to iridescence:

1. Interference: The effect of light entering the layers of aragonite, bouncing around, and some of it coming back out to the viewer. The colour of iridescence in Tahitians has been found to be pretty much totally dependent on average aragonite layer thickness over the depth of the nacre (from surface to nucleus), and has been modelled pretty thoroughly! In this pic the number is the average aragonite layer thickness over the depth of nacre:


And the colours correspond kind of *amazingly* well to what you actually see IRL!! In the pic below A and B are actually opposite sides of the same pearl, with A corresponding to tablet layer depth of ~325nm and B corresponding to depth of ~350nm.


One of the articles referenced in post #4 actually explicitly states that interference doesn't contribute to body colour, but given that interference is a study of how light responds to having moved through layers and layers of aragonite and conchiolin - I don't see how that's possible!


2. Diffraction grating: Parallel grooves created by aragonite layer edges. Higher groove density (lots of grooves packed tightly together), and smoother groove surfaces, and regular spacing between groves all yield stronger iridescence.



So in this paired pic (resolution is 10µm),
A = Pearl has strong iridescence. The grooves are about 3.5µm apart, and are very smooth and evenly-distributed and cleanly parallel.
B = Pearl has weak iridescence. The grooves are about 11.5µm apart and are irregularly-spaced and not really parallel to each other.

And in the alien green pic below (resolution is 100µm, a bit less zoome in!) the edges of the aragonite layers are super wavy, spaced far apart, and haphazard, and this pearl has no diffraction component to its iridescence at all.


So you know how when you're looking at two pearls, and one pearl's halo seems to shift as you rock the pearl but never really changes colour, and the other pearl's iridescence actually changes colour depending on what angle you're looking at it from? So seems like the first pearl's iridescence is probably created mostly by interference whilst the second pearl's iridescence probably has a big diffraction grating component.

Here's a fun graph of how the iridescence colour created by diffraction grating changes depending on what angle you're viewing the pearl from If you looked at this particular chunk of nacre straight on face-up, 0 degrees from the normal, you'd see mostly green (peak at about 550nm). If you left the nacre chunk where it is and moved your head off the normal you'd start to see other colours - red, blue, violet.



- - - - - - - - - -

Fan, Zhou, Myagkaya. (2021) Investigation of nacre nanostructure by analyzing its structural color pattern
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8490444/#MOESM1

Tan, Wong, Lee. (2004) Iridescence of a shell of mollusk Haliotis Glabra
https://opg.optica.org/oe/fulltext.cfm?uri=oe-12-20-4847&id=81312

Liu, Shigley, Hurwit. (1999) Iridescence color of a shell of the mollusk Pinctada Margaritifera caused by diffraction
https://opg.optica.org/oe/fulltext.cfm?uri=oe-4-5-177&id=63375

 

[yssie]

Why do some pearls show more iridescence colours than others?

Ignoring strength of iridescence.

- - - - - - - - - -

So... Ignoring any diffraction, considering only the interference component...

I wouldn't have guessed this, but apparently for a given total nacre depth, the *average* nacre layer thickness is basically all that matters for primary iridescence colour. As in - the main colours of that pearl's iridescence will be the same whether consecutive nacre layers are all about the same depth or whether there's quite a bit of variance in thickness - whatever the variance averages out to.

But if consecutive layers are of varying thicknesses the iridescence will show fewer colours:

The corresponding phase difference variance should average out upon superposition, so the hue of the interference color should not be affected, while […] chroma might be reduced a little

A simulation of visible akoya pearl colour according to aragonite tablet thickness - same idea as the Tahitian colours chart in Post #5, just specific to (unbleached and unpinked, presumably!) akoya:



- - - - - - - - - -

Fan, Zhou, Myagkaya. (2021) Investigation of nacre nanostructure by analyzing its structural color pattern
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8490444/#MOESM1

Ozaki, Kikumoto, Takagaki, Kadowaki, Odawara. (2021) Structural colors of pearls
https://www.nature.com/articles/s41598-021-94737-w

 

[yssie]

Why do some pearls show stronger iridescence than others?

Ignoring the specific iridescence colours.

- - - - - - - - - -

This one was really interesting.

Here's a circled Tahitian pearl. The paper looks at what's going on in three different parts of this pearl - labelled I, II, and III. And this pearl's nacre happens to not have the groove density or groove organization to support any diffraction grate component to its iridescence.


These are pictures of the iridescence at II and III - the iridescence is much stronger at II than III!


And here's what's actually going on in those regions of the pearl. At II (left below), the ratio of aragonite to conchiolin is very high, and light really doesn't penetrate very deep into the nacre. At III (right below) the ratio of aragonite to conchiolin is lower (there's more conchiolin, relatively), and light penetrates further into the nacre. Turns out more light penetration into the nacre causes lessened saturation of iridescence, because of the increase in destructive wavelength interference. The authors of this particular paper CYA this with the word "possibly", but other sources are happy to cite definitive and causal correlation... And it does kind of intuitively make sense, right? (Though I guess we all know the dangers of that! )

greater conchiolin index means slower energy decay of light when it impinges in the repetitive double layers, so higher proportion of incident light could reflect back from deeper layers. The longer the light waves travel, the more likely they become out of coherent length. As wavelength independent reflection mainly adds lightness to the color, a greater conchiolin index possibly results a less saturated color.

- - - - - - - - - -

 

[yssie]

When you've got a baroque pearl, or a circled pearl, why do the pointier/more rounded parts of the pearl often seem to be more strongly iridescent and more colourfully iridescent than the flatter parts of the pearl?

Open question for me still.

- - - - - - - - - -

This one's got me questioning my own question. I definitely see... More glowiness in the curvier parts of baroque pearls. But what am I actually looking at...?

So the most simplistic answer goes back to diffraction grating - for two parts of a pearl with the exact same quality of nacre (same nacre tablet shape and size and arrangement, same conchiolin index, same total nacre depth), a tighter corner probably means a tighter diffraction grate, with higher density of grooves. Which means stronger iridescence (due to diffraction).


But the pointer parts of baroque pearls glow more even when there's no multicoloured iridescence. Why!? This paper explains how there are three components to how light interacts with the pearl surface - reflection, transmission, and scatter. But what's the impact of curvature, exactly...?



- - - - - - - - - -

Fan, Zhou, Myagkaya. (2021) Investigation of nacre nanostructure by analyzing its structural color pattern
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8490444/#MOESM1

Ozaki, Kikumoto, Takagaki, Kadowaki, Odawara. (2021) Structural colors of pearls
https://www.nature.com/articles/s41598-021-94737-w

 

[yssie]

Why do some pearls have glossier surfaces than other pearls? Specular reflection - that high-gloss lacquer surface...

And how do some pearls manage super strong "metallic-ness"/iridescence without super high gloss?

Another open question from me.

- - - - - - - - - -

So Japanese farms harvest akoya in winter.

Why?

Because it's cold, and the pearls are most lustrous when it's cold.

Why?

...

Some WSS and GSS are metallic but not glossy, and some WSS and GSS are glossy but not metallic. And some Tahitians have strong iridescence but kinda matte surfaces, and some Tahitians are glossy but non-iridescent.

Why?

... And this is about where I start going in circles.


What I know:

1. Oysters secrete nacre more slowly when it's cold. So total nacre depth created in the cold months is less than total nacre depth deposited when it's warm.
2. Aragonite tablets change shape and size with depth. I assume this shift is a function of both temperature and pressure, because when you go deeper into water temperature drops and pressure increases, but from reading the article (first one linked below) about five times I'm still not totally sure that's what they're getting at.

3. Aragonite layers aren't ever totally parallel - layers of aragonite tablets don't stack on top of each other totally perpendicular to the nucleus surface... When the tempterature is higher there's more angular variation in how the aragonite layers stack on top of each other, when temperature is lower they're deposited more perfectly parallel to each other (and more perfectly perpendicular to the normal from the nucleus).
4. "In general", tablets with more L/W surface area + cleaner tablet edges + tighter tablet organization + more parallel tablet layers + less tablet layer depth variance + higher aragonite to conchiolin ratio = more awesomer pearl in every way (more saturated body colour, glossier surface, more metallic/iridescent).

Assuming those earlier definitions of body colour and iridescence are right, or at least reasonably accurate, what needs to happen to the nacre structure to produce a pearl with a high-gloss surface but without much iridescence? And what needs to happen to nacre structure to produce iridescence without gloss?


The haze around the light spot that you always see in pearls, even awesome pearls, we always talk about how "sharper" pearls show less of that haze. And "sharper" pearls also show sharper reflections of everything else in their environments - they're glossier.

The second paper linked was written a quarter century ago. It attributes this light spot haze to reflection of light inside the nacre - light that enters the pearl reflects off each nacre layer, inside the pearl, before coming back out to the surface. And contends that more of this internal reflection means a brighter overall body, and more "luminescence" (that how-is-this-pearl-glowing-in-a-dark-room effect), but less "sharpness" (glossiness). And conversely, less light bouncing around between the nacre layers before exiting, that creates sharper reflections but less "glow".


Okay so it's not the worst explanation every. But it does beg the question of precisely what causes more or less of this nacre-internal reflection. Keeping in mind that those nacre layers might be 10-100 nanometers apart, we are not talking about a lot of space here!! And then there's also that whole edge effects thing, that definitely exists and that hadn't been discovered when this paper was written...

But the 2021 Nature paper - third link - brings up that idea of light scattering off layers within the pearl as well, and adds both light scattering off the nucleus and light transmitting into the nucleus to the picture:

Light in a pearl is scattered at boundaries of layered structures in nacre and nucleus due to nonuniformity of the layered structures.

Sooooo...

If more nonuniformity between consecutive nacre layers (varying depths and angles of tablet layers stacked on top of each other) = more internal light scatter,
And
If more internal light scatter = less glossiness,
Then
More nonuniformity between consecutive nacre layers would mean less sharp reflections.
But more nonuniformity between consecutive layers also means less rainbowy iridescence.
So that means that a not-glossy pearl with super rainbowy iridescence shouldn't be possible. My Sea of Cortez pearls definitely missed that memo.

If a higher conchiolin to aragonite ratio = more internal light scatter,
And
If more internal light scatter = less glossiness,
Then
A higher conchiolin index would mean less sharp reflections.
But a higher conchiolin index means less saturated iridescence.
So that means that a not-glossy pearl with super strong iridescence shouldn't be possible. But I've got Tahitians and South Seas that fit exactly that profile!

So it seems like the problem is the more internal light scatter = less glossiness assumption. If glossiness was disconnected from internal light scatter, if glossiness is wholly a function of direct reflection, and if direct reflection is wholly dependent on only the outermost layers of nacre... Now there's an explanation for glossy plastic-ey pearls and non-glossy soap-bubble pearls: The outermost layers of nacre "count most" toward sharpness of reflection but all layers of nacre count equally toward iridescence/"metallic-ness".




But this bit - direct reflection is wholly dependent on only the outermost layers of nacre - where's some explanation for that? Just how deep can a "direct reflection" penetrate before it's lost to scatter? I'm out of search ideas.

- - - - - - - - - -

Rousseau, Rollion-Bard. (2012) Influence of the Depth on the Shape and Thickness of Nacre Tablets of Pinctada margaritifera Pearl Oyster, and on Oxygen Isotopic Composition

https://www.researchgate.net/publication/231589973_Influence_of_the_Depth_on_the_Shape_and_Thickness_of_Nacre_Tablets_of_Pinctada_margaritifera_Pearl_Oyster_and_on_Oxygen_Isotopic_Composition

Dobashi, Nagata, Manabe, Inokuchi. (1998 ) Implementation of a pearl visual simulator based on blurring and interference

https://www.researchgate.net/publication/221019596_Implementation_of_a_pearl_visual_simulator_based_on_blurring_and_interference

Ozaki, Kikumoto, Takagaki, Kadowaki, Odawara. (2021) Structural colors of pearls
https://www.nature.com/articles/s41598-021-94737-w

Salman, Stifler, Shahsafi, et. al. (2021) Hyperspectral interference tomography of nacre

https://arxiv.org/pdf/2010.08170.pdf

Shi, Jin, Liang. (2013) Study of the surface microstructure and optical properties of pearls induced growing by rare earth cerium

https://www.tsijournals.com/articles/study-of-the-surface-microstructure-and-optical-properties-of-pearls-induced-growing-by-rare-earth-cerium.pdf

 

[Daisys and Diamonds]

Mate i love this thread
but i gotta have a cup of tea and a lie down
This is a fantastic thread but im going to pass out in a minute from lack of oxygen or something
Ill study one post in depth each day

 

[yssie]

More questions I would love thoughts on!!

- Why do some akoya have hammered surfaces?
- What's the difference between overtone and orient?

 

[CMN]

I recent thread has just sparked some questions in my mind so I am submitting them here.

What is the safest way to clean pearls. Does chlorine in tap water damage pearls and if so why. Also, does boiling the tap water remove this risk and if so how?

 

[CMN]

Sorry @yssie I just realised I have breached rule number one of this thread to only submit one question per post. But my questions are interrelated so….I hope I‘m not banned

 

[yssie]

I just realized I broke my own rule earlier too

I’m curious to see if there’s a definitive answer to your cleaning question! I’m also wondering what that other thread will converge on… I personally boil to get rid of chlorine, and my water is really hard and boiling definitely *softens* it but I haven’t actually thought about why!

 

[Pearlescence]

We've been faced by all manner of dirty pearls in for a re-string, from the farmer's wife who wore them every day all day, including for milking etc, to the package which stank of perfume (and not so nice heavy perfume at that)
They all get a simple wash with ordinary liquid soap in the sink. Just lather up your hands and then swoosh the strand in your palms. Rinse and repeat if necessary. Put the plug in.
If you are the sort who produces a lot of skin oil and therefore gunks up your pearls and stringing, ask for a synthetic thread next time. It won't mind getting wet while it does the silk no good at all.

 

[springerspaniel]

What creates a pearl's body colour?

Ignoring iridescence! And also ignoring treatments.

- - - - - - - - - -

Both pigments in the nacre (pigment binds to the conchiolin protein matrix, not the aragonite) and the structure of the nacre itself.

The metabolism (quantity and quality of nacre secretion) of the pearl sac depends on the environment that the graft recipient/host oyster provides. The host oyster is responsible for the nacre structure of a pearl, which translates directly into pearl quality - size, luster, blemishing, nacre thickness, smoothness of skin. The donor oyster, on the other hand, is primarily responsible for a pearl's pigmentation.

A younger host (12-24 months), regardless of donor age
- Can provide a more nutritious and protective environment, which *in general* produces a higher-quality pearl
- Is more likely to die from the implantation procedure than a larger (older) oyster

An older/second cycle host, regardless of donor age
- Is larger, and can handle a larger nucleus, yielding a bigger pearl
- Is more likely to survive the implantation process
- But slower metabolism means less nacre production

Using a younger donor (12-24 months), regardless of recipient age
- Is more likely to yield a lustrous pearl
- If we're talking about Tahitians, is more likely to yield a green-bodied pearl

Using an older donor (>24 months), regardless of recipient age
- If we're talking about Tahitians, is more likely to yield a grey pearl

So that's pigmentation and, y'know, the oyster getting it's generic annual physical.


Then there's the the nacre structure component. The first paper I'll link is a seminal work that's been referenced a gazillion times - it investigates how nacre structure itself impacts body colour when there's no pigmentation involved.

In this pic you're looking at a cross-section of a bunch of aragonite tablets - imagine if you cut straight into a pearl from surface down to nucleus.


In this pic the bricks are the sides (depths) of those aragonite tablets. So to cut from surface down to nucleus, you'd cut top left to bottom right of the pic. And the little rectangle top left is a closeup of the space between two layers of aragonite tablet. The paper calls that space the "edge band", and the totality of the effect of all of those "edge bands" throughout the nacre all over the pearl is the "edge effect".


The theory proposed in this paper - which everyone seems to agree with, even if some seem somewhat reluctant - is that how clean the edges of the tiles are, and how regular the spacing of the tiles is, is what determines what the body colour of a pearl will be in the absence of any pigmentation. Unbroken tiles, clean tile edges, an organized matrix of tiles with regular spacing results in a more saturated body colour, with the colour itself being determined by how wide the edge band is. The cool part is that they've (successfully!) modelled the correlation between edge band thickness and pearl colour!! If the edge band is ~75nm wide you get a silver pearl. If the edge band is ~90nm wide you get a gold pearl. And if the tiles are broken up, misformed, have raggedy edges, aren't organized well - you get a less saturated (and less "metallic") body colour.



My understanding is that the impact of these edge effects is basically totally outstripped by presence of any pigmentation. So with a Tahitian pearl... The body colour is whatever colour the pearl's pigment is, and this edge effect has negligible impact on that body colour.

- - - - - - - - - -

Snow, Pring, Self, Losic, Shapter. (2004) The origin of the color of pearls in iridescence from nano-composite structures of the nacre
http://www.ruf.rice.edu/~rau/phys600/p1353.pdf

Ky, Demmer, Blay, Lo. (2017) Age-dependence of cultured pearl grade and color in the black-lipped pearl oyster Pinctada margaritifera.
https://www.researchgate.net/public...ck-lipped_pearl_oyster_Pinctada_margaritifera

Le Moullac, Schuck, Chabrier, Belliard, Lyonnard, Broustal, Soyez, Saulnier, Brahmi, Ky, Beliaeff. (2018 ) influence of temperature and pearl rotation on biomineralization in the pearl oyster, Pinctada margaritifera.
https://journals.biologists.com/jeb...nfluence-of-temperature-and-pearl-rotation-on

Blay, Planes, Ky. (2018 ) Optimal age of the donor graft tissue in relation to cultured pearl phenotypes in the mollusc, Pinctada margaritifera.
https://www.researchgate.net/public...notypes_in_the_mollusc_Pinctada_margaritifera

Ky, Pabic, Koua, Molinari, Nakasai, Devaux. (2015) Is pearl colour produced from Pinctada margaritifera predictable through shell phenotypes and rearing environments selections?
https://archimer.ifremer.fr/doc/00285/39637/38129.pdf

Mamangkey, Agatonovic, Southgate. (2010) Assessing Pearl Quality Using Reflectance UV-Vis Spectroscopy: Does the Same Donor Produce Consistent Pearl Quality?
https://www.researchgate.net/profil...t-Pearl-Quality.pdf?origin=publication_detail

THIS. IS. AWESOME.
THANK you so much @yssie!!

 

[Pearlescence]

As to pearl colour...how the colour happens is scientifically known and straightforward.
The real question is why? Why do molluscs make shells (and therefore pearls) with coloured insides. No mates, no light..no point. But they do. I've asked evolutionary scientists and they say that it is still a puzzle.

 

[yssie]

As to pearl colour...how the colour happens is scientifically known and straightforward.
The real question is why? Why do molluscs make shells (and therefore pearls) with coloured insides. No mates, no light..no point. But they do. I've asked evolutionary scientists and they say that it is still a puzzle.

I would argue that the fact that people in 2022 are still writing papers on how colour is expressed in pearls suggests that it’s not straightforward at all I feel like I’ve got the 101. Would need more physics and maths review than I care to do to go further.
But your question of why oysters bother in the first place is really interesting. I’ve been thinking about it for the past day and I honestly haven’t a clue. What are the running theories, if any!?

 

[yssie]

I asked Takahashi about the hammered skins on some akoya…



Yy explained that it’s called “Shi wa” in Japanese, and that it actually appears in all types of pearls! But it’s most visible in akoya because akoya have such thin nacre, comparatively.

And I know for fact that these hammered pearls can be super contrasty and iridescent, no deleterious side effects. Yy mentioned - and Western vendors have also observed - that this skin texture will keep a pearl from the topmost quality band but outside of that it has less effect on pricing than other quality factors (like blemishing).

Any tradepeople have insight into what causes this?

 

[Pearlescence]

I would argue that the fact that people in 2022 are still writing papers on how colour is expressed in pearls suggests that it’s not straightforward at all I feel like I’ve got the 101. Would need more physics and maths review than I care to do to go further.
But your question of why oysters bother in the first place is really interesting. I’ve been thinking about it for the past day and I honestly haven’t a clue. What are the running theories, if any!?

How the colour happens is pretty much clear (there may be fine details but it is a matter of more research)..
But it is clear that there is no reason whatsoever for any colour. It's dark where most molluscs live (and they have no eyes to see it anyway) and they certainly aren't making colour to attract a mate. As I said I consulted a super-top evolutionary biologist who fired up his planet sized brain and his friend's brain too and they had no suggestions.

 

[Daisys and Diamonds]

How the colour happens is pretty much clear (there may be fine details but it is a matter of more research)..
But it is clear that there is no reason whatsoever for any colour. It's dark where most molluscs live (and they have no eyes to see it anyway) and they certainly aren't making colour to attract a mate. As I said I consulted a super-top evolutionary biologist who fired up his planet sized brain and his friend's brain too and they had no suggestions.

Sometimes nature / God just like to surprise us
Maybe its one of earth's mysteries we are not surposed to know ?

who wants to live in a world without a little bit of magic ?

 

[MrsBlue]

No reason for color is the best reason. I'd hate for everything in the world to be utilitarian. Pandas can walk around not blending into darn anything.


Can we talk about keshi? Are keshi truly never nucleated or is this another problem we need to file under our "the pearl industry needs to get its crap together" list?

 

[yssie]

Why do some pearls have glossier surfaces than other pearls? Specular reflection - that high-gloss lacquer surface...

And how do some pearls manage super strong "metallic-ness"/iridescence without super high gloss?

Another open question from me.

- - - - - - - - - -

So Japanese farms harvest akoya in winter.

Why?

Because it's cold, and the pearls are most lustrous when it's cold.

Why?

...

Some WSS and GSS are metallic but not glossy, and some WSS and GSS are glossy but not metallic. And some Tahitians have strong iridescence but kinda matte surfaces, and some Tahitians are glossy but non-iridescent.

Why?

... And this is about where I start going in circles.


What I know:

1. Oysters secrete nacre more slowly when it's cold. So total nacre depth created in the cold months is less than total nacre depth deposited when it's warm.
2. Aragonite tablets change shape and size with depth. I assume this shift is a function of both temperature and pressure, because when you go deeper into water temperature drops and pressure increases, but from reading the article (first one linked below) about five times I'm still not totally sure that's what they're getting at.

3. Aragonite layers aren't ever totally parallel - layers of aragonite tablets don't stack on top of each other totally perpendicular to the nucleus surface... When the tempterature is higher there's more angular variation in how the aragonite layers stack on top of each other, when temperature is lower they're deposited more perfectly parallel to each other (and more perfectly perpendicular to the normal from the nucleus).
4. "In general", tablets with more L/W surface area + cleaner tablet edges + tighter tablet organization + more parallel tablet layers + less tablet layer depth variance + higher aragonite to conchiolin ratio = more awesomer pearl in every way (more saturated body colour, glossier surface, more metallic/iridescent).

Assuming those earlier definitions of body colour and iridescence are right, or at least reasonably accurate, what needs to happen to the nacre structure to produce a pearl with a high-gloss surface but without much iridescence? And what needs to happen to nacre structure to produce iridescence without gloss?


The haze around the light spot that you always see in pearls, even awesome pearls, we always talk about how "sharper" pearls show less of that haze. And "sharper" pearls also show sharper reflections of everything else in their environments - they're glossier.

The second paper linked was written a quarter century ago. It attributes this light spot haze to reflection of light inside the nacre - light that enters the pearl reflects off each nacre layer, inside the pearl, before coming back out to the surface. And contends that more of this internal reflection means a brighter overall body, and more "luminescence" (that how-is-this-pearl-glowing-in-a-dark-room effect), but less "sharpness" (glossiness). And conversely, less light bouncing around between the nacre layers before exiting, that creates sharper reflections but less "glow".


Okay so it's not the worst explanation every. But it does beg the question of precisely what causes more or less of this nacre-internal reflection. Keeping in mind that those nacre layers might be 10-100 nanometers apart, we are not talking about a lot of space here!! And then there's also that whole edge effects thing, that definitely exists and that hadn't been discovered when this paper was written...

But the 2021 Nature paper - third link - brings up that idea of light scattering off layers within the pearl as well, and adds both light scattering off the nucleus and light transmitting into the nucleus to the picture:


Sooooo...

If more nonuniformity between consecutive nacre layers (varying depths and angles of tablet layers stacked on top of each other) = more internal light scatter,
And
If more internal light scatter = less glossiness,
Then
More nonuniformity between consecutive nacre layers would mean less sharp reflections.
But more nonuniformity between consecutive layers also means less rainbowy iridescence.
So that means that a not-glossy pearl with super rainbowy iridescence shouldn't be possible. My Sea of Cortez pearls definitely missed that memo.

If a higher conchiolin to aragonite ratio = more internal light scatter,
And
If more internal light scatter = less glossiness,
Then
A higher conchiolin index would mean less sharp reflections.
But a higher conchiolin index means less saturated iridescence.
So that means that a not-glossy pearl with super strong iridescence shouldn't be possible. But I've got Tahitians and South Seas that fit exactly that profile!

So it seems like the problem is the more internal light scatter = less glossiness assumption. If glossiness was disconnected from internal light scatter, if glossiness is wholly a function of direct reflection, and if direct reflection is wholly dependent on only the outermost layers of nacre... Now there's an explanation for glossy plastic-ey pearls and non-glossy soap-bubble pearls: The outermost layers of nacre "count most" toward sharpness of reflection but all layers of nacre count equally toward iridescence/"metallic-ness".




But this bit - direct reflection is wholly dependent on only the outermost layers of nacre - where's some explanation for that? Just how deep can a "direct reflection" penetrate before it's lost to scatter? I'm out of search ideas.

- - - - - - - - - -

Rousseau, Rollion-Bard. (2012) Influence of the Depth on the Shape and Thickness of Nacre Tablets of Pinctada margaritifera Pearl Oyster, and on Oxygen Isotopic Composition

https://www.researchgate.net/publication/231589973_Influence_of_the_Depth_on_the_Shape_and_Thickness_of_Nacre_Tablets_of_Pinctada_margaritifera_Pearl_Oyster_and_on_Oxygen_Isotopic_Composition

Dobashi, Nagata, Manabe, Inokuchi. (1998 ) Implementation of a pearl visual simulator based on blurring and interference

https://www.researchgate.net/publication/221019596_Implementation_of_a_pearl_visual_simulator_based_on_blurring_and_interference

Ozaki, Kikumoto, Takagaki, Kadowaki, Odawara. (2021) Structural colors of pearls
https://www.nature.com/articles/s41598-021-94737-w

Salman, Stifler, Shahsafi, et. al. (2021) Hyperspectral interference tomography of nacre

https://arxiv.org/pdf/2010.08170.pdf

Shi, Jin, Liang. (2013) Study of the surface microstructure and optical properties of pearls induced growing by rare earth cerium

https://www.tsijournals.com/articles/study-of-the-surface-microstructure-and-optical-properties-of-pearls-induced-growing-by-rare-earth-cerium.pdf

I was searching for info on shi wa and got derailed. Word to the English-speaking wise, don't google "hammered skin" "nacre" on your work phone

Some additional explanation for "chalkiness", which... I guess we can call it the opposite of gloss? The difference between these two pearls, for example - grabbed this pic from one of the papers I'll link below.




Some quotes (I'm removing citations etc. for readability here):

- - - - -

Cultured saltwater pearl production involves two oysters: a donor and a host. Small pieces of the mantle—the organ responsible for shell formation in molluscs—are excised from the donor oyster and surgically inserted into the gonad of the host, along with a spherical bead known as the nucleus. Over time, the explanted mantle grows around the nucleus to form a continuous epithelial layer, the pearl sac. The pearl sac first secretes an organic layer onto the surface of the nucleus. This is followed by the deposition of successive layers of calcium carbonate, first prismatic and then nacreous, although a large degree of variation can be observed in individual pearls. This structural layering is similar to that observed within the pearl oyster shell that also consists of three layers; an outer organic-rich layer (the periostracum), a middle prismatic layer of calcite, and an inner nacreous layer of aragonite. These similarities have led to the generalized assertion that pearls are essentially inverted shells.

Temporal transcriptomic analysis has further revealed that SMPs associated with the prismatic shell layer are up-regulated in the early stages of pearl formation, whereas those associated with the nacreous shell layer are up-regulated later, suggesting that the molecular process of pearl formation largely recapitulates that observed in the shell.



- - - - -

Scanning electron microscopic (SEM) imaging obtained from the cross section of pearls revealed two distinct pearl layers with clear cut differences in microstructures that were visible on the surface of the nucleus. Microstructural analysis additionally demonstrated that an initial organic layer was deposited onto the nucleus surface before the secretion of prism and nacre (Fig. a, b). It was also noticeable that the thickness of organic material was variable among different pearls and even in different parts of the same pearl (Fig. a, b). A heterogeneous prismatic layer in contact with the initial organic layer was then accumulated onto the nucleus before the secretion of the outer aragonite nacreous layer. Unlike mollusk shell, prismatic layer in pearl was diversified. The overall composition of the epithelial secretion during the formation of prismatic layer is variable among the pearls (Fig. a, b). There was considerable diversity in the structure of the prismatic layer compared to the regular brick-wall like structures of nacre (Fig. a, b). Surface structure of 3 months pearls further suggested the significant increase in nacre in the form of aragonite crystals towards the maturation of the nacreous layer (Fig. b).


SEM images illustrating microstructural characterization of the pearl layers at (a) 1 month and (b) 3 months of grafting. nu: nucleus, o: organic layer indicated by black arrow, p: heterogeneous prismatic layer, na: nacreous layer. Uppercase letter A-D indicates different pearls. Scale bars: 10 μm

- - - - -

A is a closeup of the aragonite layer structure and B is a closeup of the calcite prismatic layer structure in Tahitian pearl oysters.

- - - - -

Takeaways - no new news here but in the interests of documentation -

Immediately after implanation of nucleus and donor tissue, the host oyster secretes a thin organic layer over the nucleus. The host then deposits calcite on top of this organic material - the totality of these deposits is called the calcite prismatic layer... And nacrous aragonite layers follow.

Pearls with thinner prismatic layers are glossier and more iridescent than pearls with thicker prismatic layers. Pearls with no calcite prismatic layer at all - that is, nacrous aragonite deposited directly onto that initial organic layer - are the glossiest and most lustrous of all. And presence of/thickenss of that prismatic layer is determined by (and can now *kind of* be predicted by) gene expression, which is ultimately governed by
A) Donor age and health,
B) Precisely which part of the donor mantle was used,
C) Implanter skill - minimizing trauma to the host oyster
D) How old the host oyster is, and
E) Host genomics black magic.

There's a fantastic thread on the p-g forum, I can't link it here so I printed the thread as a PDF and I'm attaching it to this post - Question about cultured pearl composition.

- - - - - - - - - -

Murr, Ramirez. (2012) The Microstructure of the Cultured Freshwater Pearl
Attached as PDF

McDougall, Aguilera. (2021) Pearl Sac Gene Expression Profiles Associated With Pearl Attributes in the Silver-Lip Pearl Oyster, Pinctada maxima
https://www.researchgate.net/public...n_the_Silver-Lip_Pearl_Oyster_Pinctada_maxima

Sato, Inoue, Ishikawa, Ishibashi, Obata, Aoki, Atsumi, Komaru. (2013) Pearl Microstructure and Expression of Shell Matrix Protein Genes MSI31 and MSI60 in the Pearl Sac Epithelium of Pinctada fucata by In Situ Hybridization
Attached as PDF

Mariom, Take, Igarashi, Yoshitake, Asakawa, Maeyama, Nagai, Watabe, Kinoshita. (2019) Gene expression profiles at different stages for formation of pearl sac and pearl in the pearl oyster Pinctada fucata
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6434816/pdf/12864_2019_Article_5579.pdf

Liu, Sato, Komaru, Atsumi, Awaji, Yamamoto, Higuchi, Iwahashi, Nagai. (2017) Characteristics of pearls produced by implantation of different mantle area pieces
https://www.semanticscholar.org/pap...Sato/728e5b7ec1819bc07eb278e0f62b2b81dbc58cb7
(Google Translate... Um, y'know, works, kind of... )

Marie, Joubert, Tayale, Zanella-Cleon, Belliard, Piquemal, Cochennec-Laureau, Marin, Gueguen, Montagnani. (2012) Different secretory repertoires control the biomineralization processes of prism and nacre deposition of the pearl oyster shell
https://www.pnas.org/doi/10.1073/pnas.1210552109

Muhammad, Atsumi, Sunardi, Komaru. (2017) Nacre growth and thickness of Akoya pearls from Japanese and Hybrid Pinctada fucata in response to the aquaculture temperature condition in Ago Bay, Japan

https://www.researchgate.net/publication/316535173_Nacre_growth_and_thickness_of_Akoya_pearls_from_Japanese_and_Hybrid_Pinctada_fucata_in_response_to_the_aquaculture_temperature_condition_in_Ago_Bay_Japan

 

[yssie]

I asked Takahashi about the hammered skins on some akoya…



Yy explained that it’s called “Shi wa” in Japanese, and that it actually appears in all types of pearls! But it’s most visible in akoya because akoya have such thin nacre, comparatively.

And I know for fact that these hammered pearls can be super contrasty and iridescent, no deleterious side effects. Yy mentioned - and Western vendors have also observed - that this skin texture will keep a pearl from the topmost quality band but outside of that it has less effect on pricing than other quality factors (like blemishing).

Any tradepeople have insight into what causes this?

Every single paper and book says something to the effect of "beaten/hammered/shark skin surface has been attributed to rapid nacre growth" and "indicates increased nacre thickness". Best I can find the origination of these assertions is Elisabeth Strack (fingers crossed I've finally gotten my hands on her Pearls book at a not-intolerable price)... But someone who already has her book checked it and observed that there's no more detail there.

I found a more recent reference to one experiment that included two samples that showed this beaten surface - but they decided that those two specimens weren't worth investigating further.

UGH. What the heck IS this!?

 

[LilAlex]

Thank you, @yssie -- this is such an amazing resource!

 

[Pearlescence]

Can we talk about keshi? Are keshi truly never nucleated or is this another problem we need to file under our "the pearl industry needs to get its crap together" list?

Keishi have no nucleation by definition. The only reason they are not classed by CIBJO as natural pearls is because of the difficulties association with origin. At the moment they are a serendipitous by-product of cultivation but if they were classified as naturals, with the concomitant value boost then farmers would be finding ways to trigger them

 

[Starstruck8]

Thank you @yssie for this. It's a wonderful resource.

I have to say I'm not really into pearls, but I am into geekery, and this is seriously geeky...

One of the articles referenced in post #4 actually explicitly states that interference doesn't contribute to body colour, but given that interference is a study of how light responds to having moved through layers and layers of aragonite and conchiolin - I don't see how that's possible!

This has me puzzled too. Snow et. al. seem to say that the gold body colour of GSS pearls is caused by interference in the boundary zone between nacre plates (the conchiolin in the gap itself and its extension into each plate). They say the the nacre plates themselves are too thick to give strong interference. But Fan et. al. say that iridescence colours are caused by just such interference, and they have photos that illustrate it (Figures 2b and 3C). So I'm confused. Granted, Snow et. al. are talking about body colour, and Fan et. al. are talking about iridescence. But on Snow et. al.'s view, are they really distinct?

And here's what's actually going on in those regions of the pearl. At II (left below), the ratio of aragonite to conchiolin is very high, and light really doesn't penetrate very deep into the nacre. At III (right below) the ratio of aragonite to conchiolin is lower (there's more conchiolin, relatively), and light penetrates further into the nacre. Turns out more light penetration into the nacre causes lessened saturation of iridescence, because of the increase in destructive wavelength interference.

As I read the article, the ratio they mention is the ratio of the refractive indices. (This is explicitly stated in the supporting information - top of last page.) A higher ratio of refractive indices gives stronger reflections at each interface, so the light doesn't penetrate as far into the nacre. That said, I have trouble seeing why the refractive index of the conchiolin would vary.

The real question is why? Why do molluscs make shells (and therefore pearls) with coloured insides. No mates, no light..no point. But they do. I've asked evolutionary scientists and they say that it is still a puzzle.

My totally uninformed guess is that it may not be adaptive at all. It just happens that some molluscs lay down their nacre in plates of a very uniform thickness that's just right to cause interference in visible light. Note that there a plenty of molluscs that don't show pretty colours. We have just chosen the ones that do.

 

[MrsBlue]

Keishi have no nucleation by definition. The only reason they are not classed by CIBJO as natural pearls is because of the difficulties association with origin. At the moment they are a serendipitous by-product of cultivation but if they were classified as naturals, with the concomitant value boost then farmers would be finding ways to trigger them

I have a special love for knobbly, droopy, baroque, and circled pearls but keshi are my favorite. To me, the light play on uneven surfaces is magical.

 

[yssie]

I have to say I'm not really into pearls,

I said that too!
But I'm thinking... If you're already reading journal papers you're already at the bottom of that rabbit hole, might as well take a look around whilst you're down here anyway right?

This has me puzzled too. Snow et. al. seem to say that the gold body colour of GSS pearls is caused by interference in the boundary zone between nacre plates (the conchiolin in the gap itself and its extension into each plate). They say the the nacre plates themselves are too thick to give strong interference. But Fan et. al. say that iridescence colours are caused by just such interference, and they have photos that illustrate it (Figures 2b and 3C). So I'm confused. Granted, Snow et. al. are talking about body colour, and Fan et. al. are talking about iridescence. But on Snow et. al.'s view, are they really distinct?

I totally agree. It just doesn't make sense to attribute body colour to "edge band" effects and at the same time contend that those edge band effects don't affect iridescence colour. So this is just my opinion... But I've mentally mostly put it down to Snow et. al. writing in 2004, which seems to be before a lot of studies specifically about the effects of nacre layer thickness were done.

Nothing that I've encountered that's been written since Snow et. al. has actively tried to disprove their conclusions - like, everyone afterward seems to take for fact that tablet alignment and the edge space between tablets will have a quantifiable and predictable impact on body colour in the absence of pigment. But later studies have also pretty dang conclusively proven that nacre layer thickness drives the interference component of iridescence...

Y'know. One thing I can say for sure is that over the past several months I've noticed a trend: Papers on pearls that were written "early" - before 2000ish, say, often take a very maths-first approach. Find a mathematical explanation and retrofit visible pearl behaviour. If that makes sense? Whereas articles written after maybe 2010ish go more toward put the pearl under a microscope and then try to explain what we're seeing. I don't know if that shift is a consequence of better imaging becoming more accessible, or the pearl farming industry changing, or academia trends changing, or maybe a lessening of cultural mores that discouraged questioning authority figures and established expertise... But I think Snow et. al. might have been one of the first groups to take that examination-first approach. And I think one of the biggest reasons their paper is so often referenced is that they probably actually inspired future experimentation!

There's more stuff from 2021 and 2022, some of it looks interesting and relevant, but I can't get to it because I refuse to pay

 

[yssie]

As I read the article, the ratio they mention is the ratio of the refractive indices. (This is explicitly stated in the supporting information - top of last page.) A higher ratio of refractive indices gives stronger reflections at each interface, so the light doesn't penetrate as far into the nacre. That said, I have trouble seeing why the refractive index of the conchiolin would vary.

Ooh. WAIT up. I just had a thought. Hang on rereading.