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Ana Vasiliu Blog

Pearls of many layers

Ana Vasiliu

Pearl ahead of nacre tide. The borderline bisecting the image N–S is the edge of a thick nacre layer, once tenuously kept away from the white flatland ahead of it by a living muscle barrier… Between muscle and shell the small pearl pictured became embedded into the shell. Many more small pearls are encased in this shell - a nice photomicrography subject. Such natural pearls are sometimes retrievable in full - a somewhat unusual technique as sample preparation goes [advice particularly welcome]. From my point of view, embedded pearls are very useful: they come with a full record of where they have formed over the mollusk's body - most free pearls are anybody's guess in this regard. Many have a good blister cover as well, in this case, the blister is a fine layer of perfectly transparent prismatic aragonite that has 'disappeared' in transmitted light; the small ripples of light around the pearl are typical for this material.  (Photo: Ana Visiliu • Sample: Pteria sterna shell, from Perlas del Mar de Cortez)

Pearl ahead of nacre tide. The borderline bisecting the image N–S is the edge of a thick nacre layer, once tenuously kept away from the white flatland ahead of it by a living muscle barrier… Between muscle and shell the small pearl pictured became embedded into the shell. Many more small pearls are encased in this shell - a nice photomicrography subject. Such natural pearls are sometimes retrievable in full - a somewhat unusual technique as sample preparation goes [advice particularly welcome]. From my point of view, embedded pearls are very useful: they come with a full record of where they have formed over the mollusk's body - most free pearls are anybody's guess in this regard. Many have a good blister cover as well, in this case, the blister is a fine layer of perfectly transparent prismatic aragonite that has 'disappeared' in transmitted light; the small ripples of light around the pearl are typical for this material. 

(Photo: Ana Visiliu • Sample: Pteria sterna shell, from Perlas del Mar de Cortez)


In the last post (please see the caption attached to three images on Pinctada maxima pearls—here) I wrote that finding nacreous pearls formed next to or within molluscan muscles is quite surprising, since muscles are involved in making a different kind of material to which they attach: the prismatic aragonite mentioned in the caption, not nacre. Of course, by 'surprising' I mean 'well worth looking into'. Now that I did look, I am founding what I have expected rather than anything surprising: my pearls embedded in the muscle attachment area are of prismatic aragonite.

Please see below some such 'prismatic aragonite' formed in the muscle attachment area (marked 'A' on the first picture below) and on a pearl (marked 'B'):

Click to enlarge.

Click to enlarge.

Fine aragonite prisms on the edge of a shard from area A:

Pearl SEM

The pearl in 'B' detached, together with some shell around it. Part of a relatively thick layer of the pearl was broken away, so you can see its section exposed. The first image shows the spot in the second:

Pearl Photomicrograph

An aside: there might not be any nacre on this one pearl, but nacre and prismatic aragonite layers do mix interestingly in this area of the shell. 

As said, there are myriad tiny pearls embedded in this shell! Quite a few visible outright, ever more appearing with a bit of magnification… And this, aside from the 80-something free pearls found all over the mantle—which are nacreous. Four of them were broken to inspect their cores: these proved to have fairly different from one another, but none contained any fibrous aragonite. 

Yes, I would certainly wish to know what each of these or those pearls is made of… Frustratingly, the embedded pearls are not so easy to extract from the shell. If I were to pursue all (a couple of weeks of marathon microscopy, give or take), the shell would be reduced to small, useless shards itself… Taking a look is simple fun, getting useful information out of pearl destruction is fun, but not simple—the subject of a couple of papers in the works.

Returning to the P. maxima pearls in the GIA photos, I wonder if they may have started forming with the material that muscle can produce—prismatic aragonite, like my P. sterna in the electron-micrograph, then, turned to nacre, much like Jameson found to be possible. Does it matter? If fine nacre pearls with relatively large, dense mineral cores of fibrous aragonite do appear on the market, perhaps… Else, I now have this fantastic example of transition between two forms of aragonite documented in the muscle attachment of Pteria sterna, if not in any of its pearls. 

Have I found any pearls with prismatic aragonite? Yes. Some with prismatic aragonite spherulites as cores (Pinctada radiata), another with prismatic aragonite layers interspersed among its nacre (Mytilus). The latter kind of defect is reported in shell nacre as well. Sum: there are a few ways to get prismatic aragonite in natural pearls, aside from the muscle connection. In all these cases I am finding no great gap between the nacre and the prisms, especially, no organic deposits marking this transition that may stand out in non-destructive imaging of natural pearls: