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Ok, revisiting the OP, this: ------------------------------------------------------------------------------------------------------------------------------------------------------------ Similar to Bivalvia, shell structures underlying the myostracum layer in the Polyplacophora, Gastropoda...

Again agreed mainly because of the conflation between the two terms. The burning question being, which came first, the chicken or the egg. We also agree on the majority of cases where once mythical non-nacreous objects are indeed nacreous in terms of structure. Clams pearls for certain. Even...

Perhaps, but if nacreous goes poof, so does non-nacreous by default. We build dams from concrete and steel. Beavers build dams from sticks and mud. Nature builds dams from rocks. Let's say I built a dam from a random material... lets say glass, does this mean it's no longer a dam but something...

Nacre is primarily an immediate barrier between hard and soft tissues. Secondarily, once laid up by successive layers it's becomes part of a structure. Two markedly different roles in biology, where one morphs into another. We ought not ignore the root cause to suit the overall effect.

And that's a fair point, which would imply nacre which is non-terraced/columnar. I would still submit all are orthorhombic terminations in a proteinaceous matrix to the same end... nacre.

That would be a huge range over the phyla of the animal kingdom. At the end of the day it doesn't matter because lowly nacreous is still nacreous. I agree we can split hair when it comes to the degree or arrangement of aragonite present in fewer species. I still assert cross-lamellar is nacre...

Yes, absolutely. Interestingly enough, the aformentioned quote suggests a difference between cross-lamellar and nacreous, but only to the degree of structure, not finish. I would still assert the final step in the third phase is nacreous. The aragonitic platelet model is not exclusive to nacre.

Crossed-lamellar is an elegant termination of aragonite between the Myo/periostracial and quiescent phases, thus interfacing the prismatic and nacreous phases to a great degree. The movement between these phases are gradual, not abrupt. In fact, this interlocking is the basis for exceptional...

A very important point. Nacreous is a growth phase. It occurs between the prismatic and quiescent phases nine to eleven times a year according to lunar periods, hence layered. This phase involves elegant termination of calcium carbonate, whatever that may be. Although one could, there is no...

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...

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...

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...

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...

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...

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...

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...

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

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...

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.

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...