Q&A: Pearlfacts For Not-Pearlologists

[MrsBlue]

Perhaps Darwin was wrong.

Human eyes come in different colors and there's no particular reason for that either, that I'm aware of. Now I've managed to disparage Darwin along with the poor mollluscs.

Also, I hate all of you for sparking my sudden interest in oyster reproduction.

 

[Pearlescence]

Errr yes there is. Good ol Darwin again. It's down to the melanin in the eyes, which is a mix of genetics and a response to ambient conditions - like skin colour.

 

[MrsBlue]

Errr yes there is. Good ol Darwin again. It's down to the melanin in the eyes, which is a mix of genetics and a response to ambient conditions - like skin colour.

I thought you were asking the why of color and not the how.

Early humans all had dark eyes but I suppose some mutations happened along the way resulting in lighter eye colors. So there's no particular reason why eyes should be blue or green. They just happened and evolution didn't select against the trait. The same could be true of pearl colors. There's no selective pressure against color so it can evolve freely,

But for the how of color I found this paper. Not sure if it's been linked already but I found it super interesting. Does anyone know if farms already have established genetic groups of oysters in hopes of shooting for pearls with more predictable colors? We does this with everything else that's farmed so it makes perfect sense.

Molecular Pathways and Pigments Underlying the Colors of the Pearl Oyster Pinctada margaritifera var. cumingii (Linnaeus 175

https://hal.archives-ouvertes.fr/hal-03178866/document

 

[Pearlescence]

That's not it at all. Melanin is produced by a body in response to light. So over generations the bodies which produced more melanin in regions with more sunlight did better. They had dark skin and dark eyes. In areas nearer the poles there's less sunshine and we have pale skins so we can make more vit D (which the body can't store) In scotland it is routinely added to foods because the country is just far enough north and cloudy enough that the population don't manage enough D in the winter.
Now we have sun cream and airplanes so everything is mixed up

 

[Starstruck8]

So here is a question from a non-pearlologist.

'Ringed' pearls are common, so is ringed iridescence. How does this come about? I'm guessing that the 'oysters' must be spinning the pearls as they grow. But why and how do they do this?

 

[yssie]

So here is a question from a non-pearlologist.

'Ringed' pearls are common, so is ringed iridescence. How does this come about? I'm guessing that the 'oysters' must be spinning the pearls as they grow. But why and how do they do this?

I had the same question a little while ago

Lemme find info. Somewhere on here.

 

[yssie]

I had the same question a little while ago Lemme find info. Somewhere on here.

Okay I guess I didn’t actually post on PS. Or if I did I didn’t cite anything. Here’s my layman’s summary from some of my notes. Looking forward to your take, might be more recent refs too
- Pearls spin inside the pearl sac as nacre deposited layer after layer
- Spin begins on sac formation and continues for lifetime of pearl development
- Spin in random directions creates spherical pearls
- Spin about a single axis (or a small few similar axes) yields baroque/circled/shapes - longest direction of the pearl indicates axis about which the pearl was spinning
- Pearls spinning randomly turn more slowly than pearls spinning about a single axis
- Temperature doesn’t affect spin direction or speed (but hugely affects nacre deposition rate)
- Spin direction is defined by initial calcite deposition pattern (how? Referenced paper is old and not free) and influenced by “defects” (unpredictable irregularities) as layers are deposited

That’s for the actual body shape though. The round pearls with splotchy/zoned iridescence… Or near-round pearls with uneven iridescence that *isn’t* patterned around the longest axis… Good question

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

 

[yssie]

For anyone who’s interested in the #WhyColours convo - the papers that @lissyflo and @MrsBlue posted seem to be two sides of the same coin, so many authors in common! And they’re both great reads, even for someone without any relevant background like me.

- - - - - - - - - -

I read MrsBlue’s recommendation first. The colour of the inner shell of a Tahitian pearl oyster matches the colour of that oyster’s pearl, and in this study the authors identified some different biologic processes (pathways) that yield five Tahitian colours - red, yellow, green, black, and white:


They actually found that all Tahitians are "polychromatic", meaning that all Tahitian oyster shells do have all pigments for all colours. It’s just that some specimens express one (or more) pigment in much higher proportion than the other(s), so they show more of one (or more) colour. The only oysters that don’t have all pigments are albinos (whites) - those oysters have no pigment at all. And the fact that all non-albino Tahitian oysters are actually polychromatic makes breeding for specific colours that much more complicated.

The red colour is attributed to:

Dysfunctions of Porphobilinogen Deaminase in the Heme Pathways Produce Red Uroporphyrin and its Derivates in Red Individuals

Paraphrasing Wikipedia - there’s apparently something called a porphyrin, which is some sort of organic #thing that hemoglobin needs to function. Hemoglobin links to these porphyrins and binds iron and carries oxygen to your tissues. Don’t know anything about how or why. But apparently… Sometimes something goes wrong and too much porphyrin (uroporphyrin) is produced. A genetic problem. Uroporphyrin is one type, and it’s pretty toxic, and it’s excreted (through human urine, and into mollusc shells) - when too much of it is excreted it makes both human urine and oyster shells red. Hopefully I didn’t just make the medical folks here wince too much

The yellow colour is attributed to:

Heme, Raper–Mason, and Purine Metabolism Pathways Produce Bilirubin, Pheomelanin, and Xanthine Pigments Underlying the Yellow Phenotype

A) the gene encoding a UDP-glucuronosyltransferase (UGT), the enzyme that degrades bilirubin into bilirubin diglucuronide (a colorless compound) was found under-represented in the yellow phenotype compared with the green and red, suggesting a possible accumulation of bilirubin in the animal, later excreted and deposited into the shell.
B) The second identified pathway that may lead to the production of yellow pigment was the purine metabolism pathway, ending with the production of xanthine
C) L-DOPA, which is transformed into dopaquinone by the activity of tyrosinase-related protein 1 (Tyrp1). Dopaquinone could either become the black pigment eumelanin, through a non-enzymatic process, or the yellow pigment pheomelanin […] Raman spectroscopy results also indicated the presence of black eumelanin with a strong degree of confidence in the green phenotype (several signatures), while this was almost undetected in the yellow shells

In summary, we can conclude that the yellow phenotype results from the production and accumulation of several pigments, mainly bilirubin, xanthine, and pheomelanin (Figure 7). These three pigments come from three different pathways, which increase the potential for high phenotypic variation among individuals of this color phenotype

The green colour is attributed to:

Green pigments are uncommon in animals and usually originate from three non-exclusive sources: a mixture of black and yellow pigments, a mixture of blue and yellow pigments (hypothetically), or, more rarely, from an actual green pigment

A) Blue + yellow pigments:
blue pigment phorcabilin is mixed with yellow pigments to produce a green color. The biosynthesis of phorcabilin takes place in the heme biosynthesis pathway, resulting from the oxidation of protoporphyrin IX into pterobilin, followed by its conversion into phorcabilin by a non-enzymatic process

B) Black + yellow pigments:
L-DOPA, which is transformed into dopaquinone by the activity of tyrosinase-related protein 1 (Tyrp1). Dopaquinone could either become the black pigment eumelanin, through a non-enzymatic process, or the yellow pigment pheomelanin […] Raman spectroscopy results also indicated the presence of black eumelanin with a strong degree of confidence in the green phenotype (several signatures), while this was almost undetected in the yellow shells

C) Green pigment:
the direct synthesis of a green pigment, biliverdin, is a good candidate

So, um, yeah, I don't understand too much of that ^^ But I'm sure some folks here do.

- - - - - - - - - -

Lissyflo’s recommendation kind of picks up where MrsBlue’s stops. MrsBlue’s explains what biologic pathways yield which pigments; lissyflo’s explains why those biologic pathways happen at all.

So this is the first time I’ve heard of “epigenetics”. I had to look this up - according to https://medlineplus.gov/genetics/understanding/howgeneswork/epigenome/

Epigenetics is the study of how cells control gene activity without changing the DNA sequence […] Epigenetic changes are modifications to DNA that regulate whether genes are turned on or off. These modifications are attached to DNA and do not change the sequence of DNA building blocks. Within the complete set of DNA in a cell (genome), all of the modifications that regulate the activity (expression) of the genes is known as the epigenome.

A common type of epigenetic modification is called DNA methylation. DNA methylation involves the attachment of small chemical groups called methyl groups (each consisting of one carbon atom and three hydrogen atoms) to DNA building blocks. When methyl groups are present on a gene, that gene is turned off or silenced, and no protein is produced from that gene.

And this paper specifies that

Here, we use the term epigenetics to describe any changes in DNA methylation that occur upon environmental cues.

So quoting this paper again - the intro, specifically:
Two individuals are needed to produce a pearl, a donor and a recipient oyster. The sacrificed donor is used to provide a piece of mantle (the biomineralizing graft) which is placed, together with a small marble of nacre, into the gonad of the recipient oyster. Donor oysters are selected for their inner-shell color, since the color of a cultured pearl is determined by that of the donor oyster. Recipient oysters are selected for their vigor.

While the genetic determination of this color has been partly demonstrated several environmental factors are also known to affect the shell color, such as the depth at which an oyster is grown […] In this latter work, authors demonstrated that the transplantation of oysters from the sub-surface (−4 m) to the bottom of the lagoon (−30 m) significantly darkens the inner shell color compared with oysters maintained at the sub-surface. This induced phenotype was persistent through time (“enduring”), even after the oysters were returned to shallow water, which would seem to indicate an epigenetic control mechanism rather than a direct environmental influence acting on the darkening of the shell color.

In addition to a darkening of the coloration, previous experiments have shown that a variation in depth also affects the shape and size of the aragonite tablets of the shell of P. margaritifera. Variation in the size of these tablets could therefore lead to a variation in the quantity of pigments that can be viewed through the last biomineralized aragonite layers. Interestingly, among the genes displaying methylation changes in response to a variation in water depth, two are well-known actors of the biomineralization processes of the nacreous layer: perlucin and MGAT1.

- - - - - - - - - -

So… My takeaways:

1. When oysters live deeper in the water, nacre gets darker. And this change is “sticky” - once the oysters spend some quality time deep down under, even if they later rise closer to the surface the magic that caused that dark nacre production sticks around and dark nacre development continues...
2. That “magic” is actually “epigenetics” at work - the donor mantle determines what genes are available for expression - like, the genetic superset of every colour the pearl could be - and the host oyster’s environment (depth, temperature, food, maybe host genetics too?) alters which genes from that superset are actually expressed. Which thereby determines what colour the inner shell will actually be...
3. And the third paper I linked, it's older, it outlines how aragonite tablet shape and size changes with depth but not why - turns out, ten years later, epigenetics are probably responsible for that as well!!

- - - - - - - - - -

Stenger, Ky, Reisser, Duboisset, Dicko, Durand, Quintric, Planes, Vidal-Dupiol. (2021) Molecular Pathways and Pigments Underlying the Colors of the Pearl Oyster Pinctada margaritifera var. cumingii (Linnaeus 1758 )

Stenger, Ky, Reisser, Cosseau, Grunau, Mege, Planes, Vidal-Dupiol. (2021) Environmentally Driven Color Variation in the Pearl Oyster Pinctada margaritifera var. cumingii (Linnaeus, 1758 ) Is Associated With Differential Methylation of CpGs in Pigment- and Biomineralization-Related Genes

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

 

[Pearlescence]

Cultured pearls are mostly the colour of the donor, while keishi will be the colour of the host.

 

[Starstruck8]

Okay I guess I didn’t actually post on PS. Or if I did I didn’t cite anything. Here’s my layman’s summary from some of my notes. Looking forward to your take, might be more recent refs too
- Pearls spin inside the pearl sac as nacre deposited layer after layer
- Spin begins on sac formation and continues for lifetime of pearl development
- Spin in random directions creates spherical pearls
- Spin about a single axis (or a small few similar axes) yields baroque/circled/shapes - longest direction of the pearl indicates axis about which the pearl was spinning
- Pearls spinning randomly turn more slowly than pearls spinning about a single axis
- Temperature doesn’t affect spin direction or speed (but hugely affects nacre deposition rate)
- Spin direction is defined by initial calcite deposition pattern (how? Referenced paper is old and not free) and influenced by “defects” (unpredictable irregularities) as layers are deposited

That’s for the actual body shape though. The round pearls with splotchy/zoned iridescence… Or near-round pearls with uneven iridescence that *isn’t* patterned around the longest axis… Good question

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

Thank you for linking and summarizing the paper. I have only a few things to add.

The random spinning of the round pearl answers a question so basic that I had not thought to ask it: how do oysters make round pearls round? No doubt the random rotation averages out any irregularities in the pearl sac or its environment. (But note the irregular pearls also rotated randomly.)

The pearls were spinning surprisingly fast. (At least, surprising to me). For the ‘single axis’ pearls, the average rotation speed was 4.24 degrees per minute. That’s a whole revolution every 85 minutes. The ‘randomly’ spinning pearls were a bit slower, averaging 2.64 degrees per minute.

As to how the pearls are turned, I’m not much further on. This paper is referenced: Cartwright, Checa, Rousseau (2013): Pearls are self-organized natural ratchets. It’s not free, but there is an arXiv preprint (Long live arXiv! Support open access!): https://arxiv.org/ftp/arxiv/papers/1304/1304.3704.pdf

But, as Le Moullac et al. say, the proposed model does not seem to hold up. The model makes spinning a by-product of deposition. So with almost no deposition (as at high temperatures) the spinning should stop. But it doesn’t. Also, the estimated rotation speed is way too slow, about one whole turn in 10 days.

Any later paper on how the pearls are turned would surely have cited either the Le Moullac or the Cartwright paper. But Google Scholar does not show any likely candidates. So I guess it’s just not known.

 

[yssie]

Thank you for linking and summarizing the paper. I have only a few things to add.

The random spinning of the round pearl answers a question so basic that I had not thought to ask it: how do oysters make round pearls round? No doubt the random rotation averages out any irregularities in the pearl sac or its environment. (But note the irregular pearls also rotated randomly.)

To this point - this pic is an awesome visual representation of the effects of random/single-axis rotation!



Gueguen, Y., Czorlich, Y., Mastail, M., Le Tohic, B., Defay, D., Lyonnard, P., Marigliano, D., Gauthier, J.-P., Bari, H., Lo, C., et al. (2015). Yes, it turns: experimental evidence of pearl rotation during its formation.
https://doi.org/10.1098/rsos.150144

The pearls were spinning surprisingly fast. (At least, surprising to me). For the ‘single axis’ pearls, the average rotation speed was 4.24 degrees per minute. That’s a whole revolution every 85 minutes. The ‘randomly’ spinning pearls were a bit slower, averaging 2.64 degrees per minute.

I was also struck by how fast they apparently spin. And how quickly nacre is deposited -
cells of the pearl sac will deposit mother-of-pearl continuously on the nucleus, leading to the formation of the pearl by the superimposition of mother-of-pearl layers around the nucleus at a rate of 3 to 4 per day

As to how the pearls are turned, I’m not much further on. This paper is referenced: Cartwright, Checa, Rousseau (2013): Pearls are self-organized natural ratchets. It’s not free, but there is an arXiv preprint (Long live arXiv! Support open access!): https://arxiv.org/ftp/arxiv/papers/1304/1304.3704.pdf

But, as Le Moullac et al. say, the proposed model does not seem to hold up. The model makes spinning a by-product of deposition. So with almost no deposition (as at high temperatures) the spinning should stop. But it doesn’t. Also, the estimated rotation speed is way too slow, about one whole turn in 10 days.

Any later paper on how the pearls are turned would surely have cited either the Le Moullac or the Cartwright paper. But Google Scholar does not show any likely candidates. So I guess it’s just not known.

I’ve not found anything more recent or more definitive either. Not an exhaustive search on my end, but yes, totally agree that these would be referenced!

 

[Daisys and Diamonds]

Errr yes there is. Good ol Darwin again. It's down to the melanin in the eyes, which is a mix of genetics and a response to ambient conditions - like skin colour.

I do beleave it evolution and if i could have gone to university genetics was one area that fasciated me in high school biology
I read recently all blue eyed people are descended from a single blue eyed ancestor
I have green eyes myself as did both my parents, my sister has blue eyes like grandad

 

[pearlsngems]

When keishi are formed because the host ejects the nucleus, the keishi will be the color of the donor, just as they would have been if the nucleus had not been ejected.

Keishi that form when a small piece of the host's own mantle breaks off and forms a pearl sac will produce pearls the color of the host.

 

[Starstruck8]

This one may get me thrown out. As I said before, I’m not really into pearls…

As I understand lustre, a ball bearing would have perfect lustre.

But a pearl that looked like a ball bearing wouldn’t look, well… pearly. The brightness and glow that I associate with (white) pearls have to imply less than perfect contrast. (Though there could still be perfect sharpness.) So is pearl quality is intrinsically a compromise, something like lustre vs glow? Or is it that my idea of ‘pearliness’ is based on low quality pearls?

 

[yssie]

I remember the first time I saw GSS: I had Pearl Paradise put together a bracelet for my MIL with pearls the colours of her dogs. Shades of gold and brown. They found a Tahitian for the dark brown and we used GSS for the golds, and I was smitten by those golds - they looked lit from within. I remember taking the bracelet into a room that was almost totally dark and the golds still somehow sucked up the tiny bit of light that was there to do that glowball thing…

I want my dark Tahitians super iridescent and super ball-bearing-ey. Sharp. Glossy. Dark Tahitians don’t glow. But IMO akoya, South Seas, they wouldn’t be magical if they didn’t glow! I don’t have many light Tahitians, the few I do still don’t glow the way that my akoya and SS do. I think the pigment must muck with light scatter within the nacre. I’ve never seen a white/albino Tahitian in-person.

High quality white akoya tend to be very contrasty, moreso than white or gold SS. SS can be just as sharp/glossy as nice akoya, but the best SS aren’t as contrasty as the best akoya. If glow is the result of light penetrating the surface and scattering off the nucleus/inside the nacre layers…

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

If “high contrast” is created by a mirror-like surface that returns lots of incident light as a primary reflection, then there would be an inverse sort of relationship between glow and contrastiness, but I honestly don’t see that IRL. My nicest akoya look just as glowy as my nicest SS. I have seen that high contrast and glossiness don’t necessarily go hand in hand, and high iridescence and glossiness don’t necessarily go hand in hand either. High contrast seems to always correlate with strong overtone. Strong overtone being created by both interference of light passing through layers of nacre and surface diffraction grating. I personally think the glow is part of what makes a pearl look “metallic”, and so far I’ve never gone wrong selecting for high contrast in vendor photos/video as an indicator of “metallicness”, but I don’t know that that’s actually *causal* (as opposed to just unfailingly correlated).

 

[yssie]

I’m too late to edit my post but adding pics to define how I’m using those words - these are my own GSS and photos. (Is that okay in this thread? ) The right pearl is more “metallic” (more glowy, more contrasty, has stronger overtone) than the left pearl, but the left pearl is more “glossy”.







- - - - -

In these pics the big is a gem WSS and the small is a gem akoya. Both pearls are equally glossy, but the akoya is much more contrasty. And the akoya is also more “metallic” to my eyes (stronger overtone, more glow).












Most pearl vendors really do use the word “luster” to mean the kitchen sink! It’s such an umbrella term… Means everything and nothing at the same time. I get why, it’s easy for someone totally new to pearls to read and understand. Maybe once upon a time it meant something more specific.

My understanding at this point is that almost everything that contributes to “metallicness” is caused by the interaction of light with layers of nacre. “Glow” (transmission and scatter), the interference component of “iridescence” (transmission and reflection), “contrast” (which somehow varies with iridescence). The diffraction component of “iridescence” depends on outermost aragonite structure and amount of surface curvature, and that also contributes to “contrast”… And glossiness is the odd man out, dependent wholly on primary reflection off the outermost layer of the pearl.

BUT somehow glossiness and “metallic-ness” don’t seem to be inversely related IRL - high gloss doesn’t automatically mean less metallic. Which you’d think would be the case given that more reflection off the surface by definition means lower transmission into that surface!!

 

[Starstruck8]

First, I have to say that I appreciate the work you put into these posts. I also have to say that things are still far from clear. But I’m starting to see some light…

If “high contrast” is created by a mirror-like surface that returns lots of incident light as a primary reflection, then there would be an inverse sort of relationship between glow and contrastiness, but I honestly don’t see that IRL. My nicest akoya look just as glowy as my nicest SS.

I’m thinking that contrast can actually enhance perceived glow. It’s a kind of optical illusion. The dark areas make the bright areas around them look brighter. You can see this in your examples below. Between left and right, the bright reflections are equally bright (at least as photographed). But in the pearls on the right, the darker areas beside them make them look brighter (as well as making the pearls look livelier).



I think there is another factor contributing to glow. It’s mentioned in the old reference you gave:
Dobashi, Nagata, Manabe, Inokuchi. (1998 ) Implementation of a pearl visual simulator based on blurring and interference
https://www.researchgate.net/public..._simulator_based_on_blurring_and_interference
See section III: Simulation of Blurring.

What distinguishes the ball-bearing look from what I would call ‘pearliness’ is slightly diffuse reflection. The reflections are sharp, but not perfectly sharp: they have haloes and slightly soft edges. The authors attribute this to the cumulative effect of slight irregularities in the layers. You can see it in your pics. I’m thinking that the slightly soft transition gives an appearance of glow. Again, this is a sort of optical illusion. I’m not sure how it works, but it does.

I don’t have a white pearl. But the pic below of a mother-of-pearl fruit knife illustrates these points. The handle looks, well, pearly and the silver blade looks, well, silvery. Lighting was mostly from a single overhead ceiling light. The picture is deliberately underexposed, so as not to burn out the mop handle. The light rectangle at the top is matte white IRL, but grey in the pic. This shows that the bright parts of the handle, which are catching the light, are ‘brighter that white’, which I think is part of what makes ‘glow’. But the dark to light transitions are also contributing to the glow. Note that the bright parts of the blade (some of which are burned out) look bright, but they don’t glow.

And glossiness is the odd man out, dependent wholly on primary reflection off the outermost layer of the pearl.

BUT somehow glossiness and “metallic-ness” don’t seem to be inversely related IRL - high gloss doesn’t automatically mean less metallic. Which you’d think would be the case given that more reflection off the surface by definition means lower transmission into that surface!!

There is no contradiction here. Surface reflections are weaker that you might think. At an r.i. of 1.685 (for aragonite), at normal incidence, the reflected power (n.b. = amplitude squared) is about 6.5% on the incident power. This is easily enough to see, but the loss in transmitted power is barely noticeable. As an example, think of glazed white porcelain. This is as glossy as it gets, but that doesn’t make the white body look grey.

I want my dark Tahitians super iridescent and super ball-bearing-ey. Sharp. Glossy. Dark Tahitians don’t glow. But IMO akoya, South Seas, they wouldn’t be magical if they didn’t glow! I don’t have many light Tahitians, the few I do still don’t glow the way that my akoya and SS do. I think the pigment must muck with light scatter within the nacre.

Yeah, I searched on ‘ball-bearing’ and found your amazing pic of one of your Tahitians against an actual ball-bearing. Very impressive!

I suspect you are right about pigments killing glow. My guess is that if the direct reflection is not bright enough, it won’t glow. I will have to think some more about the relation between body colour, direct reflection brightness and iridescence.

You may be amused to read that I bought a (low quality) Tahitian to play with. It’s fascinating how soft light brings out the iridescence. Left pic in window light, right pic with the light softened with translucent paper.

 

[yssie]

@Starstruck8, let me thank you in return, I’m enjoying your posts here. They are making me think and question what I say. And occasionally smack myself, like just now.

I’ve got to get to work but I’ll respond properly within a day or two. Just wanted to say that before disappearing!

 

[yssie]

There is no contradiction here. Surface reflections are weaker that you might think. At an r.i. of 1.685 (for aragonite), at normal incidence, the reflected power (n.b. = amplitude squared) is about 6.5% on the incident power. This is easily enough to see, but the loss in transmitted power is barely noticeable. As an example, think of glazed white porcelain. This is as glossy as it gets, but that doesn’t make the white body look grey.

My first reaction was "a porcelain plate isn't a thin film" and then my brain woke up. Yeah. Even at extreme angles at most 6-9% of incident will be reflected, taking that 1.685 aragonite index. Which is, as you say, gonna have no real world impact on transmission. So yeah, um, ignore that spot of stupidity.

I’m thinking that contrast can actually enhance perceived glow. It’s a kind of optical illusion. The dark areas make the bright areas around them look brighter. You can see this in your examples below. Between left and right, the bright reflections are equally bright (at least as photographed). But in the pearls on the right, the darker areas beside them make them look brighter (as well as making the pearls look livelier).

What distinguishes the ball-bearing look from what I would call ‘pearliness’ is slightly diffuse reflection. The reflections are sharp, but not perfectly sharp: they have haloes and slightly soft edges. The authors attribute this to the cumulative effect of slight irregularities in the layers. You can see it in your pics. I’m thinking that the slightly soft transition gives an appearance of glow. Again, this is a sort of optical illusion. I’m not sure how it works, but it does.

I don’t have a white pearl. But the pic below of a mother-of-pearl fruit knife illustrates these points. The handle looks, well, pearly and the silver blade looks, well, silvery. Lighting was mostly from a single overhead ceiling light. The picture is deliberately underexposed, so as not to burn out the mop handle. The light rectangle at the top is matte white IRL, but grey in the pic. This shows that the bright parts of the handle, which are catching the light, are ‘brighter that white’, which I think is part of what makes ‘glow’. But the dark to light transitions are also contributing to the glow. Note that the bright parts of the blade (some of which are burned out) look bright, but they don’t glow.

I think all of this depends on exactly how one defines "glow". I understand what you're pointing out re. the "diffuse edges" in the MOP handle - clearly a different visual effect from the blade. To my eyes the handle "glows", but (A) there isn't much contrast here, and (B) if there was no ridging or surface texture to the MOP, would we still call it "glow"? Part of what my eyes are appreciating is those tendrils of shadow at the edges of the light spot/line reflection.

For sure, more contrasty = more glowy by my definition of "glow". And I'm betting everyone else's too, even if they use different words. And I think we're also agreeing that sharper reflections (what I think of as "a high gloss laquer finish") don't contribute to "glow" one way or another.

Here's another thing. It has been observed that small pearls glow more than large pearls of equal quality.

I think there may be another optical illusion going on there, besides the effects of the surface curvature and reflection edges, and even contrast alone. Example. Five snowflakes - exact same pattern in different sizes (I shrunk the biggest one down so the smaller sizes are lossy). If I bring my face close to the computer screen the biggest snowflake is kinda jarring. My eyes look at each part of the shape individually, black circle in the middle, white pentagons around it... I feel like I'm losing the forest for the trees. But the smallest one is so small that I barely see any pattern at all. The most appealing one, to me, that immediately registers as "snowflake", "bright", "pretty" is the middle size. Or maybe the second to smallest. But as I step back from the screen the larger sizes become more immediately appealing and the smallers become hazy black splodges. So maybe part of what makes smaller pearls "glow" more is how we see patterns. In less exaggerated form than this obviously but we've still got quite a gradient between say 12/6/3mm. Similar idea to diamonds, where the experts aim for longer lower halves at very large RB sizes. In which case all of us probably have optimum pearl sphere sizes that are unique to our vision... And I guess that are pretty much universally smaller than we like to wear

I read this some time ago. The definitions they use are based on a much older paper on describing wall paint finishes. So they're overdone and cumbersome, but... They discuss Absence-of-bloom gloss which I think is closest to what you're talking about re. the MOP. And Distinctness-of-reflected-image gloss will be ball bearing sharpness.

G. Mondonneix, S. Chabrier, J.M. Mari, A. Gabillon, J.P. Barriot. (2017) Tahitian Pearls’ Lustre Assessment

https://www.researchgate.net/publication/328550037_Tahitian_Pearls'_Lustre_Assessment

It’s fascinating how soft light brings out the iridescence. Left pic in window light, right pic with the light softened with translucent paper.

Lightbox pics FTW!! My "lightbox" is made of printer paper and skewers. Works great

 

[Starstruck8]

I think all of this depends on exactly how one defines "glow". I understand what you're pointing out re. the "diffuse edges" in the MOP handle - clearly a different visual effect from the blade. To my eyes the handle "glows", but (A) there isn't much contrast here, and (B) if there was no ridging or surface texture to the MOP, would we still call it "glow"? Part of what my eyes are appreciating is those tendrils of shadow at the edges of the light spot/line reflection.

Yes, it’s a fair point that the carving is contributing a lot to what I have called the ‘pearliness’. As you imply, this is interesting in itself, but maybe not so relevant to round pearls.

Here's another thing. It has been observed that small pearls glow more than large pearls of equal quality.

I think there may be another optical illusion going on there, besides the effects of the surface curvature and reflection edges, and even contrast alone. Example. Five snowflakes - exact same pattern in different sizes (I shrunk the biggest one down so the smaller sizes are lossy). If I bring my face close to the computer screen the biggest snowflake is kinda jarring. My eyes look at each part of the shape individually, black circle in the middle, white pentagons around it... I feel like I'm losing the forest for the trees. But the smallest one is so small that I barely see any pattern at all. The most appealing one, to me, that immediately registers as "snowflake", "bright", "pretty" is the middle size. Or maybe the second to smallest. But as I step back from the screen the larger sizes become more immediately appealing and the smallers become hazy black splodges. So maybe part of what makes smaller pearls "glow" more is how we see patterns. In less exaggerated form than this obviously but we've still got quite a gradient between say 12/6/3mm. Similar idea to diamonds, where the experts aim for longer lower halves at very large RB sizes. In which case all of us probably have optimum pearl sphere sizes that are unique to our vision... And I guess that are pretty much universally smaller than we like to wear

Your snowflake story about size and scale sounds good. Here’s a story along those lines: We judge the brightness of small pearls by comparison with their surroundings. The pearls are brighter than their surroundings, so they look bright. But for bigger peals, we start comparing the brightness of different parts of the pearls. So the brightest parts don’t stand out as so much brighter. (This is consistent with the observation that greater contrast goes with greater perceived brightness.) But though this story seems sorta plausible, it’s just amateur speculation.

I read this some time ago. The definitions they use are based on a much older paper on describing wall paint finishes. So they're overdone and cumbersome, but... They discuss Absence-of-bloom gloss which I think is closest to what you're talking about re. the MOP. And Distinctness-of-reflected-image gloss will be ball bearing sharpness.

G. Mondonneix, S. Chabrier, J.M. Mari, A. Gabillon, J.P. Barriot. (2017) Tahitian Pearls’ Lustre Assessment
https://www.researchgate.net/publication/328550037_Tahitian_Pearls'_Lustre_Assessment

Thank you for the reference. Crikey! If I was a bit confused before, I’m totally confused now… But seriously, their concepts and pictures raise interesting and relevant issues that I will have to think about. I’ll get back to you if I come up with anything worth saying.