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