Lustre and nacre thickness

[BrianWanes]

Hello humans! Just looking to see if I can get some insight into the correlation of nacre thickness and lustre quality in cultured pearls.

Most of the online resources I've read that explain pearl quality and grading seem to assert that in cultured saltwater pearls, a thicker deposit of nacre around the nucleus is generally associated with a higher quality of lustre in the finished pearls.

And yet, tissue-nucleated freshwater pearls, even very nice ones, mostly display a less reflective lustre than their akoya cousins. But why? I mean, they're solid nacre, literally maximum nacre thickness! So why doesn't the "thicker nacre equals better lustre" logic seem to apply?

Just a little existential pearl quandary that's been stuck in my craw for a while. Thought I'd ask some experts!

 

[CortezPearls]

There is also the issue of "nacre quality", and by this I would say that the nacre crystals are "well shaped" (as hexagonal/honey-comb shaped as possible). You can have a thick nacre coating with "badly-shaped crystals" for nacre layers and not have great luster.
So, it's a bit more complicated than just a straight or "linear" correlation, unfortunately.

 

[BrianWanes]

There is also the issue of "nacre quality", and by this I would say that the nacre crystals are "well shaped" (as hexagonal/honey-comb shaped as possible). You can have a thick nacre coating with "badly-shaped crystals" for nacre layers and not have great luster.
So, it's a bit more complicated than just a straight or "linear" correlation, unfortunately.

Thanks for the reply! And the answer does make sense, but it leads me to another question, specifically regarding lustre in freshwater pearls versus akoya.

Does the difference in lustre have something to do with a biological or environmental difference between the two mollusks that make them? For instance, I recall reading a theory that the colder waters akoya oysters are farmed in tend to slow the metabolism of the oysters, resulting in a slower and therefore "tighter" deposition of nacre. Is it something like that?

Or is it simply that lower-lustre akoya pearls just have a tougher time making it to the market? I'm just full of questions!

 

[Pearl Dreams]

You are on the right track. Chinese fwp are farmed in warmer waters than akoya pearls, and the nacre crystals are laid down faster and larger.
Think of many small pixels vs. fewer and larger pixels in an image. Which image is sharper?

 

[CortezPearls]

You are on the right track. Chinese fwp are farmed in warmer waters than akoya pearls, and the nacre crystals are laid down faster and larger.
Think of many small pixels vs. fewer and larger pixels in an image. Which image is sharper?

Love your comparisson!

 

[pearlescence]

Two completely different species of mollusc.

 

[Pearl Dreams]

Yes, the type of mollusk has to be part of the reason. South Sea Pearls and Tahitians also are cultured in warm waters, but SSP have a more satin luster (usually) while Tahitians have higher luster (usually).

 

[BrianWanes]

I appreciate all the insight!

I'm aware that different types of pearls are cultured in different animals, but chemically speaking the nacre should be nearly identical- alternating layers of crystalline aragonite and chonchiolin.

Perhaps it's just down to the way the different mollusks secrete their nacre. Or maybe those mussels just have a tougher time laying down those "tight" layers than oysters have.

Either way, thanks again for the responses, y'all

 

[jshepherd]

I appreciate all the insight!

I'm aware that different types of pearls are cultured in different animals, but chemically speaking the nacre should be nearly identical- alternating layers of crystalline aragonite and chonchiolin.

Perhaps it's just down to the way the different mollusks secrete their nacre. Or maybe those mussels just have a tougher time laying down those "tight" layers than oysters have.

Either way, thanks again for the responses, y'all

Chemically speaking the nacre is very similar, but you're right, not all nacre lays the same.

With akoya, thick nacre is very important in achieving high luster because it's a measurement of both reflected light and iridescence. You won't have light interference creating iridescence if the nacre isn't thick enough.

But it isn't just the thickness of the nacre. Akoya pearls with the highest luster will have large, thin aragonite platelets with even distribution. Even with thick nacre, the luster will be poor if the platelets are small and unevenly distributed. Thin platelets give the pearl the needed translucency, and uniform distribution of the crystals allows light to pass evenly through the layers.

When the water temperatures drop in Japan, the shells tend to produce the thinnest (while still large) aragonite platelets, which is why pearls are harvested in the winter.

 

[CortezPearls]

We have some info here: Nacre

 

[Lagoon Island Pearls]

Aragonite may or may not pass light, depending on the phase of the growth cycle when it's precipitated. Early in the cycle, bio-fluids tend to be dark and opaque, but become more translucent as the cycle wanes, thus varied.

While conchiolin determines color and luster of pearls, translucency and refraction are expressed by "prismatic" layers of calcite, known as lathes. Lathes are what give pearls their structure and volume and moreover translucency. Aragonite merely gives added strength to the structure and smoothness to the surface for physical annexation of soft tissues. The fact nacre irridescence is alluring to humans is irrelevant, however the translucency of calcite is known to be a survival mechanism. The combination of light and gravity aide molluscs in the correct orientation which maximize access to food. The volume of available light is indirectly proportional to depth of sediments. Total darkness (through closed shells) may suggest buried to deeply within sediments, thus a radical change in survival mechanisms occur.

Aragonite is not exclusive to nacre. Science contradicts itself by confusing a composite structure with a crystal habit, after all if aragonite is substituted for foliated calcite, it's still nacre by an equal process. Aragonite and foliated calcite are variations of calcium carbonate, but with elegant terminations of the orthorhomic crystal habit due to a biotic processes (as opposed to terrestrial geology/chemistry). Earth aragonite may present as white or brown and the same is true with pearls. Again, aragonite is translucent, but may be occluded by biofluids in chonchiolin, or even reverted calcite.

After all these years, I stand on the term "non-nacreous" being redundant if not entirely misleading. Common sense dictates we are not from Mars, hence there's no burden to suggest we're non-martian when identifing ourselves. Even in the case of highly calcitic mollusks, (ie) gastropods and scallops, each have adductor muscles and hinges attached to elegant, highly aragonitic surfaces. The correct usage would be a cephalapod or nudibranch have no shells, thus are non-nacreous mollusks.

Lustre is best during winter months, when calcite and biofluids are at their minimums.