Windowpane Pearls: Calcitic nacre?

[SteveM]

A contact in Indonesia has shown me a parcel of pearls from the Placuna Placenta, family Placunidae, popularly known as the 'Windowpane Oyster' for its translucent shell traditionally used as a stand-in for glass in its native breeding areas. As can be seen, the pearls appear distinctly nacreous. But they contain no aragonite whatsoever, being composed of 99% calcite and 1% organic material, accounting for color. GIA only began studying and identifiying these pearls in 2023.

Nacreous-appearing calcite has implications for other threads in this forum that have debated the definition of nacre and the significance of aragonite in the classification of pearl qualiity.

Another intriguing aspect of these pearls is that they flouresce red under UV, exactly the same as Pteria Sterna (Sea of Cortez), thus throwing a wrench into Pteria dogma as well.

Windowpane pearls appear to be uniformly small in size—think Poe Pipi—but with similar attractiveness and as worthy of jewelry application.

A new chapter for the pearl books?

 

[SteveM]

Nacreous-appearing calcite has implications for other threads in this forum that have debated the definition of nacre and the significance of aragonite in the classification of pearl qualiity.

Specifically, that the definition of 'nacre' as the iridescent result of stacked aragonite platelets is too narrow. It must rather refer to the smooth interface between shell material and mollusk. Whether iridescent, porcelanous or (in this case) metallic in appearance becomes irrelevant.

@LagoonIslandPearls argued this point to my puzzlement at the time.

 

[CortezPearls]

Find this intriguing...do you have a Placuna shell with you? Would it be possible to use your UV lamp on it?
If it does glow pink-red then this genus also has the same -or similar- porphyrins in its shell...if not: these pearls could belong to a Pteria species and not Placuna.

 

[SteveM]

Find this intriguing...do you have a Placuna shell with you? Would it be possible to use your UV lamp on it?
If it does glow pink-red then this genus also has the same -or similar- porphyrins in its shell...if not: these pearls could belong to a Pteria species and not Placuna.

Just seeing this. The GIA report linked above does include images of Placuna pearls as well as shells under UV.

 

[CortezPearls]

Just seeing this. The GIA report linked above does include images of Placuna pearls as well as shells under UV.

Got it! Just downloaded the file...I was traveling and with very baaaaad internet!

 

[SteveM]

Find this intriguing...do you have a Placuna shell with you? Would it be possible to use your UV lamp on it?
If it does glow pink-red then this genus also has the same -or similar- porphyrins in its shell...if not: these pearls could belong to a Pteria species and not Placuna.

Nothing at all on the internet to confirm porphyrin pigment in Placuna Placenta. But at just 1% organic shell composition it wouldn't be much.

 

[SteveM]

Attaching detailed studies of Placuna Placenta: 2012 on the microstructural and optical qualities of the shell and 2022 on comparative genetics. Somewhere in this forest of data one would hope something pertaining to this species' red UV shell flourescence might be found.

The SEM images do show the calcite microstructure as a layered laminate, supposedly capable of approximating the optics of terraced aragonite platelets (nacre).

 

[SteveM]

GIA was kind enough to forward query to the authors of the paper linked in the OP above. Their comments on the (1) Red UV flouresence and the (2) nacreous surface appearance of Windowpane Pearls were solicited.

To summarize:

1) That porphyrin is assumed to be the source of the red UV flouresence due to the similarity of visible and photoluminescent spectra to Pteria species.

2) That CIBJO defines nacre as aragonitic, so they will not postulate beyond the published observations.

 

[Marianne]

GIA was kind enough to forward query to the authors of the paper linked in the OP above. Their comments on the (1) Red UV flouresence and the (2) nacreous surface appearance of Windowpane Pearls were solicited.

To summarize:

1) That porphyrin is assumed to be the source of the red UV fouresence due to the similarity of visible and photoluminescent spectra to Pteria species.

2) That CIBJO defines nacre as aragonitic, so they will not postulate beyond the published observations.

This is so very interesting. Thanks for sharing, Steve!

 

[MSC]

I saw those photos as well and was close to purchasing a few out of curiosity but didn't pull the trigger. Now I wish I had. Steve, would you mind emailing me those papers? I'm having a hard time downloading them. I'll DM you my email address
I'll check to see if they're still available... it would be interesting to examine in person.
Also, if aragonite is a crystalline form of calcium carbonate (calcite), then wouldn't these fall under the CIBJO definition?

 

[SteveM]

Aragonite and Calcite are each a different form of calcium carbonate crystallization, Aragonite being denser and harder but less stable, as it tends to degrade to calcite, which is more common.

 

[MSC]

Interesting... I can see why the authors didn't want to expound on the definition.

 

[Lagoon Island Pearls]

2) That CIBJO defines nacre as aragonitic, so they will not postulate beyond the published observations.

Nacre is not exclusive to aragonite. The "ous" of nacreous means aragonite in part only. Even to the purist, nacreous "structure" is not exclusive to aragonite. In fact in most cases it's greater than 50% prismatic calcite. Understanding, of course that lathes (calcitic prisms) are integral to structure also. If we are to be purist to the extreme, we could correctly describe nacreous structures as highly calcitic too, but that's not what the gem trade wants to hear.

For the sake of discussion and the purists, let say the mere presence of "terraced aragonite" between the prismatic and proteinaceous layers is mandatory to the description. We now know nearly every bivalve and most gastropods precipitate terraced aragonite, especially at the radius of the pallial adductoral vestibles, thus "aragonitic" in structure. Even in species such as Rock Oysters (Pododesmus macrochisma) we know perisostracial pearls are foliated calcite while myostracial pearls of the same are highly aragonitic. Yet they are still erroneously defined as non-nacreous under that debunked rule.

Non-nacreous is a redundant term. If you live in a wood house with a factory built chimney, we'd pretty much never refer to it as a non-brick house unless we were speaking in context about brick houses only. Conversely, never call brick houses a non-wood house either, because most are framed with wood (and/or other materials).

On the whole, non-nacreous is an ill-informed term perpetuated by a myth. In an industry that is already rife with old wife's tales and misconceptions, experts really need to start getting it right. Clinging to as they have (frustratingly) detracts from the true value and beauty of natural pearls. It's lazy, but hey... I'm just a retired diver with a huge collection that doesn't move because of bad science and narrow minded marketing.

Nacre is a layer precipitated between soft and hard tissue irrespective of structure. Period.

 

[SteveM]

Have been hoping for your input here! This GIA exposé was a perfect followup to our discussion from last year. But the authors appear to have belatedly realized that it also puts GIA's trade-subservient posture at risk of exposure.

 

[Lagoon Island Pearls]

Have been hoping for your input here! This GIA exposé was a perfect followup to our discussion from last year. But the authors appear to have belatedly realized that it also puts GIA's trade-subservient posture at risk of exposure.

It's true. The gem labs are narrow in scope. Like archaeology, much of the scientific or monetary value is lost once the object is removed from it's situation. Other than me, I don't know anyone who undertakes peri or postmortem analysis of natural pearls. I'm able to describe with reasonable certainty the types of pearls, especially the differences between myostracial and periostracial origin. Both yield significantly different structures within the same creature. Labs have little or no methodology to determine this. Instead they rely on reverse analysis by species and misleading exemplars, not physical anatomy. In any other medical or forensic profession, this would be limiting if not unacceptable.

There is a lot of talk about parasites, grains of sand and shell damage, but pretty much nothing about autoimmunity. Nuclear objects created by anti-nuclear factors. My latest round of research has been on this topic, especially pearls relating to the heart (pericardium). I always knew this occurred, but over the years have observed some distinct patterns. While many natural pearls may occur in various places, these tend to occur within common areas, however heart pearls are always in the same place. Attached to the right valve only, medial to the hinge, inferior to the periostracum. These have no discernible nucleus other than the pearl itself. They are not blisters. They form internally, are surrounded inward toward the heart, often peduncular having reduced circumference at the base.

While little is known of this phenomena, it can be supposed to be a great part of natural pearl onset nevertheless. Many pearls form by multiple etiological factors, but this type in all probability are from a single specific set of factors. Antigens and genetic predisposition. I even have my doubts that environmental stresses are factored in the majority of cases. Most creatures in the animal kingdom can present with spontaneous auto-immune diseases.

 

[SteveM]

While little is known of this phenomena, it can be supposed to be a great part of natural pearl onset nevertheless. Many pearls form by multiple etiological factors, but this type in all probability are from a single specific set of factors. Antigens and genetic predisposition. I even have my doubts that environmental stresses are factored in the majority of cases. Most creatures in the animal kingdom can present with spontaneous auto-immune diseases.

Difficult to grasp from the outside. Is the supposition that nuclei are thus spontaneously generated, becoming pearls in whichever part of the mollusk they eventually form a cyst?

 

[Lagoon Island Pearls]

Is the supposition that nuclei are thus spontaneously generated, becoming pearls in whichever part of the mollusk they eventually form a cyst?

Yes. Anti-nuclear antibodies (ANAs) bind to cell nuclei, thus attacking one's own cells. This an entirely different process from the "bridging" effect of epithelial perforations and also unlike parasitic infections. Both of which are generally localized and almost always septic. Lesions with ANAs are transient and sterile. A whole other class of pearl onset, unlike the one's we know.

While ANAs appear to behave spontaneously through programmed cell death (apoptosis) and hormones, there may be, but not always environmental factors. Presuming there is, they'd be difficult to attribute directly. Allergies (for example) are rarely universal, instead across a minor percentage of any population. Even within the range of those cases, it's causes and effects are greatly varied.

I'm not 100% certain, but I posit blood borne protoza to be a factor, which could technically be lumped in with parasitic infection, albeit markedly different than what we know about external parasites boring into soft tissues or through shells.

In my candling demonstrations, most natural pearls present with some type of signature at the nucleus, but some have no visible nucleus at all. Thus more likely an onset from antinuclear factors.