Question about cultured pearl composition (nacre structure)

[Mostawesomecoffee]

So, obviously nacre is comprised of alternating layers of calcium carbonate (in the form of argonite) alternating with layers of biopolymers. According to the wikipedia article on nacre, argonite structures are 0.5 nanometers in thickness. This is incredibly small. When looking at radiographs of cultured pearls online, it looks as if there is a gap between the nucleus and the nacre. At first I thought that this gap was a flaw; that the nucleus was seperated from the nacre. I don't think this is the case, though. I've seen this "gap" on many radiographs.

What I'm guessing is the answer to this peculiarity is that from the start of cultured pearl formation, nacre doesn't lay down uniformly. In other words, nacre is not comprised of 0.5 nm argonite alternating with equally minescule layers of biopolymer throughout its entire thickness. It seems that a cultured pearl starts by laying down a VERY thick layer of biopolymer before it even starts to lay down argonite. Once argonite starts to be deposited, only then with there be alternating layers of argonite and biopolymer on a very small scale.

Also, in some of these radiographs, there seem to be thick "bands" of nacre, that are quite a bit thicker than 0.5 nm, since they seem to account for about a tenth of the thickness of the nacre layer. Why is this? Does this represent growth cycles, the way a tree has rings?

Finally, between some of the "bands", there seemed to be spaces. Again, I don't know if these spaces represent flaws, or if they are indicative of biopolymer deposition without argonite deposition.

Any thoughts? Thanks.

 

[Pearl Dreams]

One thing I have read is that the oysters/mussels lay down nacre faster in warm waters and slower in cooler waters...hence faster near the tropics and slower in, say, Japan.

Since seasonal water temperature changes also create a warmer/colder environment for the shell, wherever the shell is located in the world, in colder months it lays down nacre more slowly and more densely, resulting in higher luster. Which is why winter is the preferred time for harvesting the pearls.

Also, where there is a winter/summer cycle, there will be denser and less dense bands of nacre. The tree rings you referred to.

 

[Lagoon Island Pearls]

Aragonite is not the first order of epithelial function, it's the third.

Periostracum ---> Prismatic ---> Aragonitic ---> Calcitic

 

[Lagoon Island Pearls]

One thing I have read is that the oysters/mussels lay down nacre faster in warm waters and slower in cooler waters...hence faster near the tropics and slower in, say, Japan.

Yes and no. It's true that during colder water temperatures, shell growth is minimal. However, in temperate waters there is a higher presence of food during warm periods and growth is greatly accelerated, almost double that of tropical waters.

 

[Mostawesomecoffee]

so, is this seeming "gap" a layer of periostracum?

 

[Lagoon Island Pearls]

so, is this seeming "gap" a layer of periostracum?

The periostracum is the proteinaceous layer on the outside of a shell which serves as a water-tight barrier for the onset of mineralization. In pearls, it's the first layer annexed to the nucleus.

The periostracum is comprised of conchiolin.

 

[pattye]

Mostawesomecoffee,

I'm not certain anyone has mentioned it before; you likely would benefit from owning PEARLS by Elizabeth Strack, an authoritative and detailed reference book about everything pearly. Wikipedia probably isn't a particularly good source for the kind of info you are seeking.

 

[Pearl Dreams]

I agree!! Strack's book is well worth getting and would answer a lot of your questions!

 

[Mostawesomecoffee]

Aragonite is not the first order of epithelial function, it's the third.

Periostracum ---> Prismatic ---> Aragonitic ---> Calcitic

If I understand correctly, within a cultured pearl, from center to outer edge, the structure is: "Very" thick (visible to the naked eye) layer Periostracum TO alternating microscopic layers of Prismatic (isomer of calcium carbonate) and biopolymer (that appear mineralized to the naked eye) TO alternating microscopic layers of Aragonitic (isomer of calcium carbonate) and biopolymer (that appear mineralized to the naked eye) TO alternating microscopic layers of Calcitic (isomer of calcium carbonate) and biopolymer (that appear mineralized to the naked eye).

I read that "calcitic layer" is another name for the "nacreous layer".

So, my understanding is that a pearl is comprised of the same layers, in the same order, as the shell. I.e., atop the periostracum layer of a cultured pearl, there is a prismatic layer which is NOT nacre, followed by an argonitic layer which is NOT nacre, and only then followed by a nacreous layer. Is this correct? Thanks.

 

[Lagoon Island Pearls]

The term "nacreous" is a disambiguation.

Initially in science, the term nacreous was used to strictly describe terraced aragonite. In natural or cultured pearls, nacreous was used to describe surface structure, hence the term "non-nacreous" erroneously reared itself.

Since the advent of scanning electron microscopy and broader field studies, it was learned that shell bearing mollusks and annelids are all nacreous to some degree. To that end, there is no such thing as "non-nacreous" unless you strictly refer to something that has no shell. For an example of correct usage, an octopus or nudibranch are non-nacreous mollusks.

Nacre can be likened to the term "sheets" on a bed. They can be satin or they can be cotton, but they're still just sheets. In modern science, nacre for all intents and purposes is intended to mean a smooth layer where shell and tissue meet, irrespective of structure.

The term "highly" can be applied to all mollusks. For example, the Pinctada species are highly nacreous, because that third order of growth is prevalent having thin prismatic layers and thick nacre. Clams are highly prismatic, having thick shell structure, but thin nacre. Some mussels, ie galloprovincialis etc. have soft shells, hence highly proteinaceous. In fact all mollusks begin their life after metamorphosis as such, then grow accordingly.

If it was all that simple, the term nacreous would suffice, but there are notable exceptions. That being foliated calcite or columnar structures. Even where these structures predominate across the surface of the extrapallial cavity, terraced aragonite prevails, especially where the hinges or adductor muscles attach to the shell at the level of the myostracum. Hence the term "non-nacreous" cannot (even remotely) apply. However, this where we can make the distinction between periostracial or myostracial pearl origin. Cultured pearls are periostracial. As are keishi, but it's plausible a small minority may be myostracial. Natural pearls are both. In fact the finest, translucent natural pearls are largely myostracial in origin. These pearls rarely, if ever have visible nuclei. Mabe pearls are neither, but extrapallial in origin.

Lastly, the term calcitic can be applied to nacre in some cases. The third order in foliated (calcite) structures. Likewise in over-mature adults, aragonitic lay up all but ceases from e-cells in the fourth order, giving way to calcite. Nacre is not limited to mollusks. Some annelids (calcareous tube worms) have nacreous (albeit crude) shell linings.

This is why pearls are harvested during early post-juvenile growth, as opposed to leaving them extra years because the latter defeats the purposes of luster and orient. Older "highly calcareous" pearls can be peeled to reveal an otherwise high quality pearl.

 

[Lagoon Island Pearls]

To support my previous post, let us go in for a closer look.



We will use a foliated calcite pearl from a Rock Oyster, aka Jingle Shell (Pododesmus macrochisma)



To access the concentric layers, we'll need to break the pearl. Note the left side of the image, where a section is targeted for viewing.



At the lower left hand corner, we observe prismatic lathes, then terraced aragonite, then a space of nearly transparent conchiolin. Moving toward the center, we observe a transition from prismatic, to terraced then to columnar aragonite strands. Moreover, the exciting part of this view is the near immediate transition to foliated calcite at the right side of the image. Four distinct extra-crystalline structures present in a single view.

One interesting side note: Observing the margin between the columns and calcite, we see a row of extremely microscopic pearls within the structure.

A picture is worth a thousand words. Cool, eh?

 

[GemGeek]

Very cool.

 

[AMc]

^^^^ What she said!.

 

[SunSeeker]

Wow, very cool!

 

[Lagoon Island Pearls]

Two views of terraced aragonite in California mussels (Mytilus californianus) which are nearly identical to the nacreous structures of the Pinctada species. The third view is fouth order (over-mature), calcitic structure. I included the third image to demonstrate how orient and luster are diminished at senescence. The classic orthorhombic shape of aragonite morphs into randomly "oriented" calcite.

I'd be remiss not to credit the imagery to Ana Vasiliu for her boundless vision and collaborative efforts in this research.

 

[CathyKeshi]

Thank you, Dave ... fascinating!