Iridescence

[Valeria101]

I am not sure if this really is a 'resource'. Just let me know if you do not like what you see: these took my breath away!

Microscopic pictures of iridescent layers in black opal and pearl crystallites

Source:
MINDAT, Collection and photo: Albert Russ. (for opal)

Mollecular Expressions (for the pearl side )

With a teaser...

 

[knotty panda]

You always the most interesting and informative sites! I loved the bubbles. A picture is worth a thousand words.

 

[CortezPearls]

Loved the images Valeria. The one on the left seems full of "fractals"...someone should try to see if a pearl's surface grooves (its fingerprint) have a fractal interpretation. I am sure they do.

These patterns are partially responsible for iridescence in a pearl and in mother of pearl. The tighter these spirals are, the more intense the iridescence.

 

[Valeria101]

That is fantastic! The picture of the pearl fingerprint... Makes me dream of a new kind of book of the pearl - one more intimate with the nature of the beast rather then the 'glam slam' put forth by most to date.

Would it be OK to ask how was it taken (just roughly - was it a microscopic system? What magnification...).

The picture in my post wasn't even of a pearl - just recrystallized nacre. The real thing is fascinating! Especially if one relate the intimate details of nacre with the look, or species of the pearl (which most likely should be the case... I would think).

Was it a round pearl? The pattern in the picture seems rather typical for rotation - an Archimedean spiral.

 

[Valeria101]

[used to be double post, for some reasons]

Had to check!

 

[knotty panda]

Well, this is a FASCINATING discussion! I love how you graphed the spiral. Excellent question about fingerprints. If the spin is tighter, the more iridesence, does a keishi have multiple or overlapping spirals?

 

[CortezPearls]

Wow Valeria! Looks beautiful!!! Loved your idea of placing both images toghether and very nicely done as well.

Actually, it is a scanned image of a photo taken by the SSWF Gem Lab in Basel, under a electron microscope. It appears on the "Cultured Pearls from the Gulf of California" article that was published in Gems & Gemmology (http://www.gia.edu/gemsandgemology/18578/20298/2178/back_issue_article_detail.cfm) in the year 2004.

I don't know exactly what pearl was used to take that photo since we gave Lore Kieffert (the author) several cultured pearls (of inferior quality I may have to add...we could not part with the expensive ones ) and these were used for all sorts of tests.

BTW, I am writting a book (since 2004) but about the "Nature of the Beast" (well, I am a biologist...I care more about the living world than the marketing and glam aspects of the Pearl). I already have 12 chapters written but need to review this information with lots of people and when ready comes the final issue: the money to print it. Also, my twins do not allow me much respite to work on my book (but then again: I do sleep soundly at night thanks to them...the perfect solution for Insomnia: have twins!).

 

[Valeria101]

BTW, I am writting a book (since 2004) but about the "Nature of the Beast" (well, I am a biologist...I care more about the living world than the marketing and glam aspects of the Pearl). I already have 12 chapters written...

Congratulations for the twins! And the book

Your extremely photogenic pearls... also bring to mind an art book - single pearls rather then jewelry, nacre images, your stories of their biologic source and none of the dusty hearsay and (more or less) urban myths used to peddle pearls away from their source and meaning ...

If books about art can be art objects themselves, it time - I'd say - to have a book about nature that is an art object itself. And the subject is so absolutely appropriate. [sure enough, that's just me thinking out loud, nothing more]

 

[CortezPearls]

Well, this is a FASCINATING discussion! I love how you graphed the spiral. Excellent question about fingerprints. If the spin is tighter, the more iridesence, does a keishi have multiple or overlapping spirals?

Yes indeed...the tighter the nacre sutures (techical term for the lines that conform a spiral) the more iridescent the pearls are. The best patterns are seen in specimens that have superior nacre crystallization (the aragonite crystals have a more perfect haxagonal shape). According to the sources I've read (including the aforementioned article) the most compact spirals have been identified as those from the Rainbow Lipped Oyster (Pteria sterna) and some Abalone species (Haliotis spp).

The way this would work -please correct me if I am wrong, I am not a physisist- is that light will strike the spirals and these will direct light (bouncing it off) into other directions, but the spirals act as "canyons" so light is hitting and bouncing off the walls of these "canyons" in a wild manner...thus allowing for iridescence. This is basically a very simple way to explain the effect of the spirals. You also have other attributes coming into play to allow a pearl to look like a pearl, but this is one of them.

By polishing a pearl you remove these spirals...you can imagine what will happen then.

 

[SteveM]

According to the sources I've read (including the aforementioned article) the most compact spirals have been identified as those from the Rainbow Lipped Oyster (Pteria sterna) and some Abalone species (Haliotis spp).

By polishing a pearl you remove these spirals...you can imagine what will happen then.

This thread is a very welcome and substantial followup to the thread on 'Orient.' The fingerprint/spiral structure is a revelation to me (as well as reinforcement of the miraculous nature of pearls), and conforms to the view that 'orient' is as much a textural/dimensional effect as it might be iridescence. As the garish iridescence of abalone is seldom if ever referred to as 'orient' (too much of a good thing?) might there be a happy medium in terms of the compactness of the spirals?

Chinese freshwaters with their iridescence and 'orient' are almost universally polished. So is it the case that spirals on the inner nacre layer surfaces are responsible for the effect?

Steve
Seattle

 

[perlas]

Interesting. Wondering myself how much magnification it would take to see the spirals.... can't see them at 100x or maybe I have to look again.

If there are more compact spirals in the inner nacre layers of a pearl then a successful peeling will give the pearl more iridescence. Maybe polishing pearls sacrifices iridescence but enhances luster. Quite a curiousity. If only I could pearl a pearl successfully although I'm not sure if I'd sacrifice a pearl with great iridescence.

Makes sense that the compactness of the spirals or maybe grooves/waves(?) in an uneven manner (just guessing) is responsible for the play of colors in freeform baroques and abalones.

 

[Valeria101]

'CortezPearls', shouldn't the spirals correspond to individual secreting cells of the pearl sac? Just guessing... since there are no differentiated structures in that tissue...

By polishing a pearl you remove these spirals...you can imagine what will happen then.

Chinese freshwaters with their iridescence and 'orient' are almost universally polished. So is it the case that spirals on the inner nacre layer surfaces are responsible for the effect?

How about anotehr hypothesis: could these structures be layers throughout the pearl, much like wood grain is? Obviously, with a different arrangement than the directional nature of wood grain.... If so, then peelin, polishng, cutting... sections of various types and directions would each produce a different effect!

Also, not sure how much to worry about polishing as dumbing-down enhancement... if it does depend ALLOT on the 'quality' (i.e. whatever property of the nacre) of the material anyway... This is no rhetorical question: I do not know and never heard of a source doing this sort of survey - of 'polish-ability' In absence of data, I am trying a parallel with the sapphire and, say, tourmaline' heating situation: how the overall effectiveness of the treatment to boost up quantity in a certain quality range determines the tratement's impact on value... No idea with which of the cases pearls would be more alike.

Interesting. Wondering myself how much magnification it would take to see the spirals.... can't see them at 100x or maybe I have to look again.

If the pictures were taken with an electron-michroscope.. that hints at magnification definitely over 500X, which is exagerately low for any type of installation. Over 10^5 is more 'normal' for lab applications (citing colleague ).

 

[CortezPearls]

Interesting. Wondering myself how much magnification it would take to see the spirals.... can't see them at 100x or maybe I have to look again.

If there are more compact spirals in the inner nacre layers of a pearl then a successful peeling will give the pearl more iridescence. Maybe polishing pearls sacrifices iridescence but enhances luster. Quite a curiousity. If only I could pearl a pearl successfully although I'm not sure if I'd sacrifice a pearl with great iridescence.

Makes sense that the compactness of the spirals or maybe grooves/waves(?) in an uneven manner (just guessing) is responsible for the play of colors in freeform baroques and abalones.

I have been able to see the spirals with only a Jeweler's loupe (40x) and appropiate light (natural, indirect) and then...slowly move the pearl in your fingers. Using stronger magnification (dissecting microscope 100x) it looks just great. But if the pearls are polished you will most likely just find "scars": what will seem to be "plow-lines".

Peeling would be able to remove a damaged layer and uncovered a good layer (non-polished), but peeling can also damage the nacre. At least that is what we've seen.

Polishing sacrifices orient/iridescence for luster. To me, it is better to have Orient/Iridescence than high-gloss luster...I mean: what would be the difference between a highly polished piece of rounded clam-shell and a pearl? It is the play of colors what makes a pearl LOOK like a Pearl. To me, this is where the Magic Resides.

So, this is the idea: you have a beautifully iridescent pearl (orient), or shockful of Overtones...would you polish it? Nope. Never ever.
But, instead you have a pearl that is of lower quality, doesn't have any "magic" in it, would probably sell for pennies...but if you polish it, then it is shiny...you can ask a bit more for it. Then maybe just dye/bleach it, then a coating of wax/plastic...do you follow? In the end you ask for more money. You don't really care much for the pearl or the production of beautiful pearls...it is mostly about just show me the money!

 

[CortezPearls]

'CortezPearls', shouldn't the spirals correspond to individual secreting cells of the pearl sac? Just guessing... since there are no differentiated structures in that tissue...
How about anotehr hypothesis: could these structures be layers throughout the pearl, much like wood grain is? Obviously, with a different arrangement than the directional nature of wood grain.... If so, then peelin, polishng, cutting... sections of various types and directions would each produce a different effect!

Also, not sure how much to worry about polishing as dumbing-down enhancement... if it does depend ALLOT on the 'quality' (i.e. whatever property of the nacre) of the material anyway... This is no rhetorical question: I do not know and never heard of a source doing this sort of survey - of 'polish-ability' In absence of data, I am trying a parallel with the sapphire and, say, tourmaline' heating situation: how the overall effectiveness of the treatment to boost up quantity in a certain quality range determines the tratement's impact on value... No idea with which of the cases pearls would be more alike..

Hmmm...you started me thinking of actually doing a bit of studies on the mantle's live cellular structures instead of the typical tissue cross-section studies we've done before.
But yes, each layer should have its own fingerprint...different from the one on top and below that given layer. We have seen this with peeled pearls.

I don't think many people care nor know about the effects of polishing and its implication$. In my case...when I want to buy a pearl I demand non-polished pearls because I do care about this fact. I would pay 10 times less for a polished pearl: I would give it the old "Natural Pearl Factor". The same goes for artificial colors and any enhancement. To me a pearl worth $100 USD that has been polished (/10), and bleached (/10) would be worth only $0.10 USD.

But -hey!- that's me! It might sound DRASTIC but it is only my personal viewpoint. You might agree -or just partially- or not, but since the Industry is not doing anything about this the I guess it is a matter of PERSONAL CHOICE.

 

[CortezPearls]

Ouch! Already felt I stepped on some toes...

But to instead continue on the scientifical information...I have yet another photo. This one is a close-up on the fingerprint-like spirals. You will see that these suture lines are formed by the microscopic nacre crystals, and you will notice that the distance between suture lines is very small: 5 microns.

Hope you find this one of interest as well.

 

[Valeria101]

Picking three separate points from the previous posts:

I have been able to see the spirals with only a Jeweler's loupe...

That sounds like one easy diagnostic for polishing to me ... if it works on most cultured pearl type (as if there were many!). If only other 'treatments' would be as easy to spot! Are they?


PS - to whom might be concerned: I am aware that polishing is an accepted treatment by historic standards. So this is theoretical chat - in line with debating the... dunno, fascinating chemical composition of garnets or something.

I would pay 10 times less for a polished pearl...
... It might sound DRASTIC but it is only my personal viewpoint.

... together with:

Polishing sacrifices orient/iridescence for luster. [...]
So, this is the idea: you have a beautifully iridescent pearl (orient), or shockful of Overtones...would you polish it? Nope. Never ever.
But, instead you have a pearl that is of lower quality, [...] if you polish it, then it is shiny...you can ask a bit more ...

Is overtone changed by polishing too?

And one more:

Can't imagine that the majority of pearls harvests around the world would be desirable w/o polishing (i.e. the most basic treatment, washing aside!). Somehow, hope this is so... and not many pearls go through the grind unnecessarily... that would be sad.

Would you say that treatments also change the price structure (i.e. relative premium from one quality class to the next) or just pushes everything upwards, or what?

The huge quantity and diversity of more or less visibly treated pearls does seem to have affected the popularity or recognition of the natural look, but then, the price tag is pretty darn recognizable (either natural pearls, or natural color that, since the 'natural surface' / 'untreated' is hardly a label yet...).

 

[SteveM]

?since the Industry is not doing anything about this the I guess it is a matter of PERSONAL CHOICE.

Ouch! Already felt I stepped on some toes...

No damage here. As any commodity finds a wider audience, it is obliged to standardize (globalize?) for predictability of quality and supply. That's not why I fell in love with pearls, however. The democratization of pearls rings politically correct but it has its downside.

Don't particularly like Starbucks coffee, either (as a Seattlite I can say that!).

Steve
Seattle

 

[CortezPearls]

Can't imagine that the majority of pearls harvests around the world would be desirable w/o polishing (i.e. the most basic treatment, washing aside!). Somehow, hope this is so... and not many pearls go through the grind unnecessarily... that would be sad.

Would you say that treatments also change the price structure (i.e. relative premium from one quality class to the next) or just pushes everything upwards, or what?

The huge quantity and diversity of more or less visibly treated pearls does seem to have affected the popularity or recognition of the natural look, but then, the price tag is pretty darn recognizable (either natural pearls, or natural color that, since the 'natural surface' / 'untreated' is hardly a label yet...).

I can imagine (and I hope!) that TOP NOTCH pearls can be easily recognized at harvest time (we can) so they would be set aside, not to be polished.

For your next question we would need someone with expertise in the field of Akoyas and/or FWPs. My guess is that the overall price structure goes upwards after processing. Jeremy would be our man, he deals directly with the producers/processors and knows what they are expecting.

As you say, polishing is an accepted treatment. Initially done with coarse grain salt, but other methods were also known: feeding your pearls to chickens/ducks, and others. I have seen good polishing on Tahitian blacks (not over-polished, you can still see their spirals) and overtones are unnafected.

 

[perlas]

A 40x loupe? I better check again. I just haven't really thought I'd see a lot in pearls than in transparent gemstones. I'd probably get more luck in a pteria mabe to see the spirals although I do have unpolished pearls...

Yes, I think we wouldn't polish a highly iridescent pearl. But "they" would for the fact that only a small fraction of the harvent actually have iridescence and when these's pearls do, it's difficult to match --- given the bodycolor and the dash of iridescence which is unique per individual pearl.

So when iridescence is sacrificed, it is sacrificed for luster and matching. I think a batch of polished pearls is still difficult to match (at least not much to be commercially viable), thus pearls are also bleached.... and pinked....

Anyway, usually freeform baroques have higher chances to have mind-blowing iridescence. This is just a guess but there maybe the waves in freeform baroques forms an irregular spiral instead of a neat, circullar spiral... there are portions that's more compact than other areas... I hope you get my drift. Wonder what it's like magnifying abolone pearls...

 

[Caitlin]

feeding your pearls to chickens/ducks, and others.

What exactly have you heard about this? How would it be accomplished? Have you ever known anyone who did this or have you done it?

I am serious about my questions, because we have thought this would be an urban legend as chickens etc have crops which woud prevent any stonelike object from passing through the entire digestive system.

Pearls would get stuck in the crop where the grinding action with tiny rocks and peices of oyster shell etc might polish the pearl but how would the harvester get them out of the crop? And why use a chicken or fowl at all? Isn't that like burning down the house to cook the piglet? Why not just tumble the pearls with bits of oyster shell instead?