Debunking another widely held myth. Nacreous v Non-Nacreous

[Lagoon Island Pearls]

For several years, I've been vociferous to the extent a commonly used term "non-nacreous" is misleading if not erroneous.

Between 2011 and 2015, I worked with Ana Vasiliu in the microscopy lab at the University of Paleontology and Stratigraphy in Granada, Spain. Initially our work was to use an SEM (scanning electron microscope) to determine the nacreous properties of my target species California Mussel (Mytilus californianus). However, the project evolved in two directions. One being the examination of micro-pearls at their onset, (which I'll not elaborate in this article) but on the other. That being my discovery of tiny pearls in Rock Oysters (Pododesmus macrochisma).

At the time, these and numerous other species of bivalves amd gastropods were considered "non-nacreous" by scientists, identification labs, collectors and marketeers, having shells largely comprised of foliated calcite. In the evidence provided by my sampling programs, we discovered pearl onset was not limited to the pallial mantle or visceral mass, (ie) gonads, byssal glands or hearts. I had discovered pearls forming in and around the adductor muscles. Rather than periostracial, instead myostracial onset. These pearls were sterile as opposed to infectious and despite having concentric growth, presented with no visible nuclei even though we agreed a micro-minuscule nucleus exists at a cellular level.

Shown below, our first electron microscopic views of the shells of P. macrochisma revealed elegant strands of aragonite between lathes of layered prismatic and foliated calcite. From this evidence, we postulated it was likely to occur in other bivalves or gastropods. As such the term non-nacreous would be incorrect.

Our work branched off into the latter mentioned study, thus we let the former hypothesis stand on it's merits.

In recent times however, scientists from China undertook to study this further by gathering numerous samples of bivalves and gastropods including abalone, scallops, mussels and geoducks (to name a few).

Here is the paper:

Characterization of the Myostracum Layers in Molluscs Reveals a Conservative Shell Structure​

​

Needless to say, I am thrilled, for it has been scientifically revealed our assertion is true. Most if not all bivalve mollusks and gastropods are defined as nacreous, irrespective of their underlying shell structures. The irridescence and orient of commercial pearls does preclude nacreous structures in other pearls. Instead the differential terminology ought to be stated as highly nacreous v highly calcitic in structure.

 

[SteveM]

Instead the differential terminology ought to be stated as highly nacreous v highly calcitic in structure.

Fantastic post!

On the above statement, where would 'highly aragonitic' fit in? I'm thinking about non-nacreous aragonite/calcite microstructure generally referenced as crossed-lamellar and characteristic of the 'flame' phenomenon. No doubt my thinking could use updating…

 

[Lagoon Island Pearls]

Fantastic post!

On the above statement, where would 'highly aragonitic' fit in? I'm thinking about non-nacreous aragonite/calcite microstructure generally referenced as crossed-lamellar and characteristic of the 'flame' phenomenon. No doubt my thinking could use updating…

Highly aragonitic is correct in terms of shell structure where foliated calcite is not present. Perisotracial (conchiolin) --> Prismatic calcite --> Nacreous. These being the three stages of a growth period. While calcite is present in the prismatic phase of most molluscs (and pearls), it's more of an elegant termination, namely lathes as opposed to reverted or massive calcite. Highly nacreous would also be correct under this scenario because "crossed lamellar" are properties of aragonite. The confusion stemmed from the presence of aragonite being lumped in with the term nacreous as a rule, but there are exceptions. It should be based upon the occurence of a crystal habit, which is elegant orthorhombic termination of calcium carbonate, not necessarily irridescence and orient.

Think of it this way, we use a sheet on the bed for comfort to the skin. The mattress gives the bed cushion. However, if we slept on a pile sheets, we'd have a multi layered cushion of sheets but still just a single sheet for comfort to the skin.

As the layers of nacre add up, their role is changed from that of comfort into structure.

In the cases where foliated calcite is present in structures, aragonite is still present as a nacreous surface, but tends to be minor by comparasin, thus the object is highly calcitic.

 

[Lagoon Island Pearls]

The confusion stemmed from the presence of aragonite being lumped in with the term nacreous as a rule, but there are exceptions. It should be based upon the occurence of a crystal habit, which is elegant orthohombic termination of calcium carbonate, not necessarily irridescence and orient.

I should add that aragonite, especially in winter months or among other species (ie) melo melo, conch etc. can appear perfectly clear. It does not always present with irridescence and orient. Those and colour are factors of interspersed proteins, namely conchiolin.

 

[SteveM]

I should add that aragonite, especially in winter months or among other species (ie) melo melo, conch etc. can appear perfectly clear. It does not always present with irridescence and orient. Those and colour are factors of interspersed proteins, namely conchiolin.

Is this the interpretation of FW shell beads from mussel? While colorless and non-iridescent, they are regarded as nacreous and thus compatible as nuclei (although now it seems the range of successful nucleus material is wider than long thought).

Is an argument in favor of the existence of colorless (white/prismatic) nacreous pearls from colorful nacreous shells suggested?

 

[BWeaves]

I really like the clear and the green ones.

 

[Lagoon Island Pearls]

Is this the interpretation of FW shell beads from mussel? While colorless and non-iridescent, they are regarded as nacreous and thus compatible as nuclei (although now it seems the range of successful nucleus material is wider than long thought).

Is an argument in favor of the existence of colorless (white/prismatic) nacreous pearls from colorful nacreous shells suggested?

Shell beads from FW mussels are indeed nacreous. Your point on the difference in aragonitic v nacreous stands to reason.

Highly aragonitic ought to be used in a scientific context because it does not default to one aspect (iridescence and orient) while overlooking the other.

Highly nacreous in a lay context as a catch all term is appropriate because it could allude to one or the other without the need of being specific.

 

[Lagoon Island Pearls]

I really like the clear and the green ones.

Me too, while they are country mile from gem quality, they are chocked full of science and wonder. Tiny pearls reveal big secrets!

 

[Lagoon Island Pearls]

Perisotracial (conchiolin) --> Prismatic calcite --> Nacreous. These being the three stages of a growth period.

I should also add to this. It's actually four stages.

Perio/myostracial --> Prismatic --> Nacreous --> Quiescence.

In lay terms. Create a water tight barrier --> Build a structure --> Get comfortable --> Rest.

Laying up nacre is a phase which is common to most molluscs. The only exceptions being some cephalopods and nudibranchs.

 

[SteveM]

Dave,

You will be anticipating this post…

Below is a report from Nature (10/17/1912) regarding the pearl gifted to T.H. Haynes (pioneering Australian pearler) by the Sultanate of Sulu in 1884. As all such reports over the centuries of pearls from the 'pearly' Nautilus, it was porcelanous and not iridescent in appearance.

In 1924, Mr. Haynes sacrificed the by-then infamous pearl to be sectioned for a report in The Proceedings of the Malacological Society of London in their 1924-1925 volume. That image, found with the help of our U. Granada friends' connections in London, is attached. For comparison is an image of sections of Conch Pearls from a pioneering 1987 article in Gems and Gemology. As we see, both the 'Nautilus' pearl and the Conchs are concentric in growth structure, as we would expect from iridescent pearls.

As you know, GIA and International Gem Society regard Conchs and other 'porcelanous' pearls as non-nacreous, and therefore not actual 'pearls' at all.

Are we at the point of understanding that the pearls in question are in fact nacreous?

 

[Lagoon Island Pearls]

Dave,

You will be anticipating this post…

Beloiw is a report from Nature (10/17/1912) regarding the pearl gifted to T.H. Haynes (pioneering Austrailian pearler) by the Sultanate of Sulu in 1884. As all such reports over the centuries of pearls from the 'pearly' Nautilus, it was porcelanous and not iridescent in appearance.

In 1924, Mr. Haynes sacrificed the by-then infamous pearl to be sectioned for a report in The Proceedings of the Malacological Society of London in their 1924-1925 volume. That image, found with the help of our U. Granada friends' connections in London, is attached. For comparison is an image of sections of Conch Pearls from a pioneering 1987 article in Gems and Gemology. As we see, both the 'Nautilus' pearl and the Conchs are concentric in growth structure, as we would expect from iridescent pearls.

As you know, GIA and International Gem Society regard Conchs and other 'porcelanous' pearls as non-nacreous, and therefore not actual 'pearls' at all.

Are we at the point of understanding that the pearls in question are in fact nacreous?

Yes albeit not highly (at least on the surface). In every collection, I'll observe one or two percent as highly calcareous. All pearl sacs depend upon a multitude of e-cell types being present. This often varies between pearls, even in the same creature. Most can be picture perfect but others not so much. When you're handed a single pearl for examination without knowledge of it's original situation, it's a fallacy to presume the object represents the group on the whole.

To quote the author "The broad end, which apparently has been flattened...." suggests he did not observe the object in situ, otherwise he'd have most likely substituted the word "apparently" with evidently for certainty.

And beside that point, even in a single specimen case, aragonite can revert to calcite under a whole host of conditions, thus presenting otherwise.

Again, I will reiterate a previous post:

Laying up nacre is a phase which is common to most molluscs. The only exceptions being some cephalopods and nudibranchs.

Quite frankly, I challenge the GIA and the Gem Society (or anyone for that matter) to refute this assertion.

 

[SteveM]

Yes albeit not highly (at least on the surface). In every collection, I'll observe one or two percent as highly calcareous. All pearl sacs depend upon a multitude of e-cell types being present. This often varies between pearls, even in the same creature. Most can be picture perfect but others not so much. When you're handed a single pearl for examination without knowledge of it's original situation, it's a fallacy to presume the object represents the group on the whole.

To quote the author "The broad end, which apparently has been flattened...." suggests he did not observe the object in situ, otherwise he'd have most likely substituted the word "apparently" with evidently for certainty.

And beside that point, even in a single specimen case, aragonite can revert to calcite under a whole host of conditions, thus presenting otherwise.

Again, I will reiterate a previous post:


Quite frankly, I challenge the GIA and the Gem Society (or anyone for that matter) to refute this assertion.

The assertion is effectively and empirically refuted. It seems as though it comes down to who has the burden of proof (as in a court of law), the gemmological sector having reached its conclusions.

We need to organize!

 

[Lagoon Island Pearls]

The assertion is effectively and empirically refuted. It seems as though it comes down to who has the burden of proof (as in a court of law), the gemmological sector having reached its conclusions.

We need to organize!

The tenets of science provide there is no burden to refute something which does not exist.

However in this case, the burden is already laid out. There is no need to initiate a methodology, field study, peer review or publication.

Let's start here. Although not cited directly in these papers, it is a baseline. Bolding mine.

In-depth proteomic analysis of nacre, prism, and myostracum of Mytilus shell.
Liao Z, Bao LF, Fan MH, Gao P, Wang XX, Qin CL, Li XM. J Proteomics. 2015 Jun 3;122:26-40. doi: 10.1016/j.jprot.2015.03.027. Epub 2015 Apr 6. PMID: 25857279

Biological significance: In this paper, we characterized for the first time the protein set from different shell layers in Mytilus. Shell matrix proteins are the major component that controls different aspects of the shell formation process and thus a source of bioactive molecules that would offer interesting perspectives in biomaterials and biomedical fields. Our data can be used as a resource for further exploring the roles of shell matrix proteins in the deposition of different shell layers (nacre vs. fibrous prism vs. myostracum) or different polymorphisms of calcium carbonate (aragonite vs. calcite), and the identified protein set of myostracum provided candidates for studying the mechanism of adductor muscle-shell attachment.

Recovering and Exploiting Aragonite and Calcite Single Crystals with Biologically Controlled Shapes from Mussel Shells.
Triunfo C, Gärtner S, Marchini C, Fermani S, Maoloni G, Goffredo S, Gomez Morales J, Cölfen H, Falini G. ACS Omega. 2022 Nov 17;7(48):43992-43999. doi: 10.1021/acsomega.2c05386. eCollection 2022 Dec 6. PMID: 36506210

Abstract​

Control over the shape and morphology of single crystals is a theme of great interest in fundamental science and for technological application. Many synthetic strategies to achieve this goal are inspired by biomineralization processes. Indeed, organisms are able to produce crystals with high fidelity in shape and morphology utilizing macromolecules that act as modifiers. An alternative strategy can be the recovery of crystals from biomineralization products, in this case, seashells. In particular, waste mussel shells from aquaculture are considered. They are mainly built up of single crystals of calcite fibers and aragonite tablets forming an outer and an inner layer, respectively. A simple mechanochemical treatment has been developed to separate and recover these two typologies of single crystals. The characterization of these single crystals showed peculiar properties with respect to the calcium carbonate from quarry or synthesis. We exploited these biomaterials in the water remediation field using them as substrate adsorbing dyes. We found that these substrates show a high capability of adsorption for anionic dye, such as Eosin Y, but a low capability of adsorption for cationic dyes, such as Blue Methylene. The adsorption was reversible at pH 5.6. This application represents just an example of the potential use of these biogenic single crystals. We also envision potential applications as reinforcing fillers and optical devices.



Now let's move along to the paper in the OP. There are numerous citations, but for the sake of the discussion let's paraphrase specific details.

They cite:

Castro-Claros J. D., Checa A., Lucena C., Pearson J. R., Salas C. (2021). Shell-Adductor Muscle Attachment and Ca2+ Transport in the Bivalves Ostrea Stentina and Anomia Ephippium. Acta Biomaterialia. 120, 249–262. doi: 10.1016/j.actbio.2020.09.053

Shell-adductor muscle attachment and Ca2+ transport in the bivalves Ostrea stentina and Anomia ephippium

Checa A. G., Macias-Sanchez E., Harper E. M., Cartwright J. H. (2016). Organic Membranes Determine the Pattern of the Columnar Prismatic Layer of Mollusc Shells. Proc. Biol. Sci. 283 (1830). doi: 10.1098/rspb.2016.0032

Organic membranes determine the pattern of the columnar prismatic layer of mollusc shells

Then the baseline for Checa et all cited in those papers... and it's an important one for the purpose of this discussion.

Layered Growth and Crystallization in Calcareous Biominerals: Impact of Structural and Chemical Evidence on Two Major Concepts in Invertebrate Biomineralization Studies​

Pelecypods (Bivalve mollusks) belonging to the Pteriidae family, among which Pinctada, the “pearl oyster”, is the most studied. With an outer layer made of calcite prisms and a thick inner layer built by aragonite in the form of nacreous tablets, this large shell appears as a perfect illustration of the classical model of Bivalve shell, and (by extension) a model for all mollusk shells.

5. Conclusions​

1. Structural, chemical and biochemical data converge to suggest that crystallization of biogenic calcium carbonate produced by a cellular epithelium (e.g., mollusk mantles, coral basal ectoderms) occurs in organic gel layers, the thickness of which is in the micrometer range, cyclically secreted by the mineralizing organs.
2. Crystallization is thought to occur as the last step in formation of the growth layer. Simultaneously, two or three different environmental signals are recorded, according to the species-specific distribution of the distinct mineralizing areas (multiple and simultaneously active “vital effects”).
3. Shaping of the resulting polycyclic calcareous biocrystals is linked to their stepping growth mode. A growth-layer cycle comprises formation of envelopes made of insoluble organic materials acting as spatial limiting factors during the crystallization step of the biomineralization cycle.
4. As a result of this crystallization process, the mineral phases exhibit a specific reticulate pattern built by tightly packed and fused nodular units, the margins of which being marked by an irregular organic rich coating.
5. Through important variations among phyla the layered control of crystallization appear as an essential component of the “common strategy” developed by the living systems for controlling deposition of Ca-carbonates which built their mineralized hard-parts.
----------------------------------------------------------------------------------------------------------------------------------------------------

My assertion can be infered in the conclusion.

Reticulate /rĭ-tĭk′yə-lĭt, -lāt″/ adj
Relating to or being an evolutionary process that involves the exchange of genes between organisms of different species.

The paper outlines differences in structures at a considerably high level, but does not identify calcitic stuctures in complete absence of aragonitic structures in any case.

As such the term "non-nacreous" is moot and redundant.

-----------------------------------------------------------------------------------------------------------------------------------------------------

 

[SteveM]

The tenets of science provide there is no burden to refute something which does not exist.

However in this case, the burden is already laid out. There is no need to initiate a methodology, field study, peer review or publication.

Let's start here. Although not cited directly in these papers, it is a baseline. Bolding mine.

In-depth proteomic analysis of nacre, prism, and myostracum of Mytilus shell.
Liao Z, Bao LF, Fan MH, Gao P, Wang XX, Qin CL, Li XM. J Proteomics. 2015 Jun 3;122:26-40. doi: 10.1016/j.jprot.2015.03.027. Epub 2015 Apr 6. PMID: 25857279

Biological significance: In this paper, we characterized for the first time the protein set from different shell layers in Mytilus. Shell matrix proteins are the major component that controls different aspects of the shell formation process and thus a source of bioactive molecules that would offer interesting perspectives in biomaterials and biomedical fields. Our data can be used as a resource for further exploring the roles of shell matrix proteins in the deposition of different shell layers (nacre vs. fibrous prism vs. myostracum) or different polymorphisms of calcium carbonate (aragonite vs. calcite), and the identified protein set of myostracum provided candidates for studying the mechanism of adductor muscle-shell attachment.

Recovering and Exploiting Aragonite and Calcite Single Crystals with Biologically Controlled Shapes from Mussel Shells.
Triunfo C, Gärtner S, Marchini C, Fermani S, Maoloni G, Goffredo S, Gomez Morales J, Cölfen H, Falini G. ACS Omega. 2022 Nov 17;7(48):43992-43999. doi: 10.1021/acsomega.2c05386. eCollection 2022 Dec 6. PMID: 36506210

Abstract​

Control over the shape and morphology of single crystals is a theme of great interest in fundamental science and for technological application. Many synthetic strategies to achieve this goal are inspired by biomineralization processes. Indeed, organisms are able to produce crystals with high fidelity in shape and morphology utilizing macromolecules that act as modifiers. An alternative strategy can be the recovery of crystals from biomineralization products, in this case, seashells. In particular, waste mussel shells from aquaculture are considered. They are mainly built up of single crystals of calcite fibers and aragonite tablets forming an outer and an inner layer, respectively. A simple mechanochemical treatment has been developed to separate and recover these two typologies of single crystals. The characterization of these single crystals showed peculiar properties with respect to the calcium carbonate from quarry or synthesis. We exploited these biomaterials in the water remediation field using them as substrate adsorbing dyes. We found that these substrates show a high capability of adsorption for anionic dye, such as Eosin Y, but a low capability of adsorption for cationic dyes, such as Blue Methylene. The adsorption was reversible at pH 5.6. This application represents just an example of the potential use of these biogenic single crystals. We also envision potential applications as reinforcing fillers and optical devices.



Now let's move along to the paper in the OP. There are numerous citations, but for the sake of the discussion let's paraphrase specific details.

They cite:

Castro-Claros J. D., Checa A., Lucena C., Pearson J. R., Salas C. (2021). Shell-Adductor Muscle Attachment and Ca2+ Transport in the Bivalves Ostrea Stentina and Anomia Ephippium. Acta Biomaterialia. 120, 249–262. doi: 10.1016/j.actbio.2020.09.053

Shell-adductor muscle attachment and Ca2+ transport in the bivalves Ostrea stentina and Anomia ephippium

Checa A. G., Macias-Sanchez E., Harper E. M., Cartwright J. H. (2016). Organic Membranes Determine the Pattern of the Columnar Prismatic Layer of Mollusc Shells. Proc. Biol. Sci. 283 (1830). doi: 10.1098/rspb.2016.0032

Organic membranes determine the pattern of the columnar prismatic layer of mollusc shells

Then the baseline for Checa et all cited in those papers... and it's an important one for the purpose of this discussion.

Layered Growth and Crystallization in Calcareous Biominerals: Impact of Structural and Chemical Evidence on Two Major Concepts in Invertebrate Biomineralization Studies​

Pelecypods (Bivalve mollusks) belonging to the Pteriidae family, among which Pinctada, the “pearl oyster”, is the most studied. With an outer layer made of calcite prisms and a thick inner layer built by aragonite in the form of nacreous tablets, this large shell appears as a perfect illustration of the classical model of Bivalve shell, and (by extension) a model for all mollusk shells.

5. Conclusions​

1. Structural, chemical and biochemical data converge to suggest that crystallization of biogenic calcium carbonate produced by a cellular epithelium (e.g., mollusk mantles, coral basal ectoderms) occurs in organic gel layers, the thickness of which is in the micrometer range, cyclically secreted by the mineralizing organs.
2. Crystallization is thought to occur as the last step in formation of the growth layer. Simultaneously, two or three different environmental signals are recorded, according to the species-specific distribution of the distinct mineralizing areas (multiple and simultaneously active “vital effects”).
3. Shaping of the resulting polycyclic calcareous biocrystals is linked to their stepping growth mode. A growth-layer cycle comprises formation of envelopes made of insoluble organic materials acting as spatial limiting factors during the crystallization step of the biomineralization cycle.
4. As a result of this crystallization process, the mineral phases exhibit a specific reticulate pattern built by tightly packed and fused nodular units, the margins of which being marked by an irregular organic rich coating.
5. Through important variations among phyla the layered control of crystallization appear as an essential component of the “common strategy” developed by the living systems for controlling deposition of Ca-carbonates which built their mineralized hard-parts.
----------------------------------------------------------------------------------------------------------------------------------------------------

My assertion can be infered in the conclusion.

Reticulate /rĭ-tĭk′yə-lĭt, -lāt″/ adj
Relating to or being an evolutionary process that involves the exchange of genes between organisms of different species.

The paper outlines differences in structures at a considerably high level, but does not identify calcitic stuctures in complete absence of aragonitic structures in any case.

As such the term "non-nacreous" is moot and redundant.

----------------------------------------------------------------------------------------------------------------------------------------------------

The point is that nacreous refers to all mollusk shell construction resulting from an organic process preceding and directing tablet and fibre crystallization?

I just looked up etymology for nacre and among its possible origins is the Arabic naqur (hunting horn), the shape of a mollusk shell.

The interface with gemmology seems to have gone haywire the moment 'non-nacreous' began to replace 'porcelanous/porcellanous.' Yes, let's ban the former from all future discourse. But the gemmologists' insistence on iridescence as the defining quality of a 'true pearl' remains.

 

[Lagoon Island Pearls]

The interface with gemmology seems to have gone haywire the moment 'non-nacreous' began to replace 'porcelanous/porcellanous.

Yes. Terms like porcelaineous describe appearance of pearls without implying structure. It's common knowledge that ceramics are created in a kiln at a high temperature. Prefixing "non" to a word should only be used in the context of it's opposite. Even in a lay context, it's redundant to suggest non-Martians like us use a term like that in any context other than living on Earth.

This thread actually applies to the "irritant" thing too, which I also find irritating. Irritation whether rashes or scratches are musculoskeletal, not exoskeletal factors. Molluscs don't have movement in the same way humans or other animals with locomotion. In humans et al, these may give rise to infections, where myostracial pearls are sterile in almost every case. In the case of periostracial pearls, there is a grain of truth in the grain of sand thing, but only in a minority of cases. Parasitic pearls certainly are not sterile. Encapsulation is likened to bridging when shells are cracked. It's purpose is to create a water tight barrier to prevent pathogens or parasites from infiltrating soft tissues, then to minimize stresses caused by friction and pressure. In those cases, irritation is clearly a secondary if not tertiary factor. The only time it may be primary is when objects are lodged in the extrapallial space (between the shell and the mantle) as is the case with cultured "mabe" pearls. These almost never give rise to loose pearls, instead ones fused to the shell in the same manner objects can be spray painted to a wall. Hence the reason no graft tissue is used in the aquaculture process. Smooth objects are used as nuclei. Angular objects don't cause irritation insomuch as perforation of adjacent soft tissues.

The gem labs really need to revisit both of these terms.

 

[Lagoon Island Pearls]

For clarity, gastropods and pectinidae (scallops) have some locomotion, even cockles and clams have feet which allow themselves to relocate or reorient, thus irritation can be a factor but there remains the issue of irritation v perforation all the same. A clam kicking it's way around does not necessarily imply the mantle is directly involved in the action.

 

[SteveM]

Terms like porcelaineous describe appearance of pearls without implying structure. It's common knowledge that ceramics are created in a kiln at a high temperature. Prefixing "non" to a word should only be used in the context of it's opposite. Even in a lay context, it's redundant to suggest non-Martians like us use a term like that in any context other than living on Earth.

Porcelanous/Porcellanous/Porcelaineous. I was going to include the third English spelling option but was afraid of too many slashes!

In all due respect, I do not think it is clear to anyone but a biomaterial scientist (and that does not include me!) about the nature of nacre vs. porcelaineous shell material, and particularly their mutual ambiguity. I think a few more lay terms may be in order.

 

[Lagoon Island Pearls]

I think a few more lay terms may be in order.

Calcareous would be true in both scientific and lay terms, but then again to what degree?

This is why I'd suggest "highly nacreous" for bright iridescence and orient of pearl oysters, abalone etc. In gemology, calcite is good and bad depending on it's properties, so it needs prefixes to avoid confusion. Most mollusks are "highly prismatic" also. Conch pearls for example are "highly calcitic" but with "foliated calcite" which implies elegant nacre with minor aragonite as opposed to just "calcareous" which tends to suggest a greater percentage of reverted calcite is present.

Semantics aside, even calcareous in absence of a prefix is infinitely more correct than non-nacreous.

 

[SteveM]

Calcareous would be true in both scientific and lay terms, but then again to what degree?

This is why I'd suggest "highly nacreous" for bright iridescence and orient of pearl oysters, abalone etc. In gemology, calcite is good and bad depending on it's properties, so it needs prefixes to avoid confusion. Most mollusks are "highly prismatic" also. Conch pearls for example are "highly calcitic" but with "foliated calcite" which implies elegant nacre with minor aragonite as opposed to just "calcareous" which tends to suggest a greater percentage of reverted calcite is present.

Semantics aside, even calcareous in absence of a prefix is infinitely more correct than non-nacreous.

I am continuing to have difficulty with the ambiguity.

"Highly Nacreous" suggests that there are degrees of nacreous expression, such as "Moderately" or (dare I say) "Non."

Aragonite tablets/platelets arranged in terraced (Bivalvia, Cephalopoda) and columnar (Gastropoda, Cephalopoda) forms, including fossilized in both forms in the case of Ammolite, seems pretty unambiguous, both in its presence or the lack thereof.

I have always associated foliated calcite with porcelainous scallop pearls, its significant presence or influence in Conchs is new to me.

 

[Lagoon Island Pearls]

I am continuing to have difficulty with the ambiguity.

"Highly Nacreous" suggests that there are degrees of nacreous expression, such as "Moderately" or (dare I say) "Non."

Aragonite tablets/platelets arranged in terraced (Bivalvia, Cephalopoda) and columnar (Gastropoda, Cephalopoda) forms, including fossilized in both forms in the case of Ammolite, seems pretty unambiguous, both in its presence or the lack thereof.

I have always associated foliated calcite with porcelainous scallop pearls, its significant presence or influence in Conchs is new to me.

Moderately or lowly/slightly are definitely acceptable. My usage of highly was for example, not exclusivity.

Quahog (Mercenaria mercenaria) are moderately nacreous. Geoduck (Panope generosa) are slightly nacreous.

Indeed, scallops are a better example of foliated calcite structures. I stand corrected as to conch pearls, those are crossed-lamellar, thus nacreous. To what degree, I'm not certain. Moderate to high perhaps? Thanks for pointing that out.